ao smith vertex

Hey everyone. I've been a long time subscriber to Dan's site and love his weekly emails. I ran into a little heating dilema in my new house and figured this would be the best place to go.

I just moved into a new home, @ 4000 sq ft that was built @ 9-10 years ago. Unfortunately, it still has the same conventional 50 gal gas fired hot water tank that was originally installed. Besides being undersized, it's on its last leg and I'm getting the tell tale signs of emminent failure. In the house lives myself and my wife, my 3 year old daughter, and we have another little one on the way. In addition, I have a relative living in a basement apartment. As far as hot water consumption goes, we have two full baths on the second floor, 1/2 bath on the first floor, and full bath in the basement. There is a dishwasher and washing machine in both the main house and apartment. The dishwashers are Bosch which are supposed to use @ 7 gallons per cycle. The killer is the two tubs (one in the master bath and one in the basement). The master bath tub is huge and has somewhere around a 85 gal capacity and uses one of those tub filler faucets which has a very high flow rate. The tub downstairs is much smaller, but still somwhere around 40-50 gallon capacity.

Well, it's too late to make a long story short but lets just say that the second night there my wife went to take a bath and the hot water heater gave up after the tub was only 70% filled. I was originally planning on putting in an AO Smith 75gal high recovery unit but then I came across the AO Smith Vertex. I have been emailing AO Smith back and forth trying to get advice but the best I've gotten so far is probably, shoulds, and maybes... What better place to go then here to see what the experts think. So, should I put on my weight lifting belt and go for the big 75gal tank (or even bigger)..Should I consider the Vertex 100?, or should I even consider the Cyclone by AO SMith - the commercial version of the Vertex?

Thank you in advance!
Matt