Masonry Chimney Liner...Stainless Steel or Aluminum?? New NG boilers.

I would do SS. I'd probably seriously consider dual swinging baro's/spill switches, and ditch the draft hoods.
The brand's manufacturer instructions would probably be able to properly size it...along with The Wall's chimney expert @Bob Harper

Ditto Steve and Steamhead. Coal soot or oil soot will destroy aluminum liners in no time. I can't get it into the UL 1777 listing as the aluminum guys are too strong right now. Still, it's allowed for all CAT I venting. Doesn't make it a good idea. 80% furnaces will eat it up, too. No listed tees with aluminum I'm aware of--they are all listed to pull through. That means you're going to have to sweep it into the room with a sharp radius at the base. Aluminum can't take this bending the way ss can so it crushes on the inside of the curve. You can't effectively shape it with a downhill slope towards the appliance 1/4"/ LF the way you can with ss pulled through or off a tee snout. We use tees as condensate traps/ inspection ports/ sweeping ports and when a pull-through bend is too tight. You can get tee snouts with a strap. You drop the liner with the tee body attached with the snout inserted in the breaching. The body of the tee passes through the ss draw band until lined up. You reach in with several 1/4" hex extension rods with a 5/16" hex drive and tighten the snout to the tee. Then try to pitch it down a little. We use ss rigid slip sections into the snout to adjust for the depth.
Aluminum is only tested to a 100lb dead load on a 10 ft. test structure. It is exempt from the 100 stroke brush test with a tight fitting wire brush ss liners must endure. SS is tested to a 200lb dead load. The Battelle Labs research with Lawrence Berkley labs found 6006 alloy aluminum ok with CAT I venting. They were using draft hood-equipped appliances. The lack of dilution air results in significantly increased condensate with 80% furnaces. Since 80% furnaces run with higher stack temps, this increases the effect of corrosion compared to the cooler stack temps of draft hood entrainment.

I know of no B-vent expressly approved for use in a chimney that previously fired coal or oil. Even if you installed Class A insulated chimney with a 444, 430 or 304L ss outer, it would probably get eaten up in a few years.

You could do a cast in place liner but there isn't much anecdotal evidence on the long term efficacy and performance of these liner systems under these conditions. Again, approval does not equal good idea.

The GAMA tables show a 9" liner would work at 50ft. I'd have to check some other resources but keep in mind these charts would require a 20% derating if a corrugated liner was used. That's a big reason sweeps use smoothwall often, to pick up that extra capacity when diameter is limited. They flow better, are easier to install, easier to sweep, and, in most but not all cases are a better product. Just beware certain larger diameters can separate. Also, those GAMA tables are for interior chimneys. You must "engineer" the liner for exterior chimneys...

We use M-flex 316ss every day along with some others. Several really good professional grade liners available with decent tech support. They offer cheap slide rules for sizing liners: one for oil and one for gas. They're ok for a quick shot at it. Keep in mind they are already derated 20% for gas and 15% for oil. If you go with smoothwall you'll have to calculate that back in for the extra capacity.

Liner mfrs. crow about the merits of insulating liners for gas and oil even though it isn't required for the listing. All that's required is a nominal 4" solid masonry unit. A "solid masonry unit" is defined by NFPA 211 as having 75% of its mass in cross-section. A 4" CMU block counts but larger blocks do not unless the open cells are fully grouted (not just loose fill like vermiculite or perlite). Thus in an old chimney of 4" bricks, you can reline with a metallic liner as long as no more than 1" of the mortar joint is gone. Beyond that, it doesn't count even if you insulate with cementitious listed insulation such as Thermix. Those are insulators and technically lightweight insulating cement. They have not been tested or recognized as a structural repair the way certain cast in place liners, such as Guardian, have been. If your chimney is weak, a cast liner is a recognized structural repair as well as a liner listed to 2,100F at 3/4" thickness. The downsides are cost and tendency to blow out of weak walls and make a royal mess. We cast in lifts to prevent this. The cold joint is approved under the Guardian listing and reduces the hydrostatic pressure that causes blowouts.

We think of aluminum as having a curbside warranty. It's warranted until the installer leaves the curb to cash the check. Who cares if it fails? They are dirt cheap and mfrs. almost never pay claims on them. People just shrug and eat the cost of a ss liner or convert to a CAT IV condensing system and abandon the chimney.

15LF is a looong horizontal offset. The maximum horizontal run for a 9" connector is 13.5 LF. Also, at 9" it would need to be 24ga. galvy. but we would just use 26ga. rigid ss liner as the connector and you'll never have to replace it. Review the sizing charts in Ch.24 of the IRC for the connector manifold and plan well. Personally, I think the use of B-vent to push the sizing is much over rated. We use it mainly to confront clearances to combustibles (6" down to 1"). Better yet, do like we now do and just use all 'L' vent instead of B-vent even for gas. The 400 series ss liner holds up fine but doesn't transfer heat as much as aluminum so you actually get higher stack temps. L vent is also approved for B-vent applications plus it's tested at higher temps: 570F vs 550F plus a 1,400F flash fire test. Think you don't need this level of heat protection? FVIR water heaters typically discharge over 600F brand new. By the time you clog the flame arrestor wire gauze in the base with dryer lint, the stack temps can run much higher.

Try to get as much vertical vent rise off the top of the appliance as possible. Dump this into a bullhead tee with double acting barometric damper with a spill switch interlocked to the gas control. You get balanced draft, dilution air, backdraft protection, and better performance with less chance of unstable combustion all for the price of 1/2 % AFUE BS.

BTW, if you common vent oil with gas you need "primary safety controls" on both. This is how youz do it on gas appliances. You install a blocked vent switch on the oil connector interlocked to the primary.
HTH

Most liners are listed for at least 60 feet using the listed top termination plate that supports the liner from above.
The bullhead allows downdrafts to burp out of the double acting baro. instead of down into the combustion chamber where it could blow out the pilot. You could do a 'wye' but I'm thinking you're actually thinking of a siamese, which is where two pipes converge into one. A wye splits into two but we misapply that term in HVAC all the time. As any fire fighter the difference.