help from chimney experts

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Just a question about installing liners. From a recent post I made here I feel I should use a chimney liner. So far I have worked with two chimney sweeps. One guy said he runs the flex liner down and bends and feeds it through the wall to the inside of the house and installs a tee inside the house that has a trap to catch any water that may run down the liner. The other said he installs a tee in the flue and said he may have to remove material at the bottom of the chimney to get the tee to fit. Both sweeps seem to be knowledgeable one is a member of the Chimney Sweep Safety Institute of America.
Are the two different proposals both good and is there any problem with either that you can see.
I have a Buderus G115/4 ( oil ) connected to a 20 ft bottom to top 8x8 clay lined chimney 3 sides exposed to the outside. The stack temp on the Buderus measured 300 degree F. This was installed in May of this year.
Hoping someone can provide some guidance.
Appreciate your help and comments,