I traded my 08 M3 for a 07 997TT. The cars are absolutely different in terms of power delivery and feel. The M is very predictable in acceleration whereas the 997tt gets there so quick, that you have to be prepared to shift alot quicker.
I do not daily drive my 997tt nor did I the e92. However, I will say that the m is indeed the more practical of the two. I honestly miss my M for its high rev and just overall relaxed feel. However, everytime I look at my 997tt I am grinning from ear to ear.
As for carseats, the Porsche carseats seem to work the best. They are pricey but it is what it is. I just purchased a Graco comfortsport seat for my two year old. It requires removing the rear seat cushion which is held by velcro.
Good luck on your decision. Both cars are amazing and are unique from a driver feel perspective. There are a ton of guys who daily drive the 997tt and find it to be very functional.