i went from 2 MGF's to having 2 GTV's and then recently a MGTF and now i really want a GTV!! the only thing that i would say is better about the TF is the RWD fun that can be had!
From your point of view and if you want max bhp then i would get the V6 as the Twin Spark is about the same power as the VVC engine,not sure on proper figures but having experienced both cars at the same time,they are similar in performance. So going from your 225bhp to 150ish bhp would be a big loss! You are used to the power so i doubt the V6 will give you any problems,im betting it will be easier to handle than a 225bhp rwd TF!!
From other points of view,the build quality is better,you will feel like your in a proper built car,kind of like when you go from a Focus to a Golf or Audi feeling...that well put together feeling! (if you get what i mean!)
Maintenance wise i think the V6 is more reliable,but im not sure if its easier to work on...The TS suffer from cambelt failures and some like to drink oil,although iv not experienced either. Do not know about the V6 but i think the cambelt changes arent as frequent,and as they are more "specialist" cars i would of thought more enthusiasts would of kept them (compared to the TS)
Either way i think you will be surprised how nice the GTV is,i had a 97 GTV before my 2002 TF,the GTV,even though it was 5yrs older and had over 100,000miles more than the TF, just won me over! Took her over to Portugal when i lived there and it was like being a celebrity! People would literally stop and stare at it!