Go ask in the GTA lounge, you will get varying opinions. Both are fantastic cars, 147 has a slightly "younger" image (it's a hot hatch after all) whereas the 156 has a bit more of a "Q car" feel about it. I much prefer the 156 GTA interior and feel overall it's a classier package, but I prefer saloons/coupes over hatchbacks. The engines in both are fantastic and a great starting point for further mods if you want to increase the performance.
You need to budget for fitting a Q2 and different suspension dampers at the least, so make sure you take that into account. Lots of good info in the GTA lounge.