I was in a similar position to you just over a year ago. I had opted for a company car and was looking at either a GT or a 159 to replace my 147.
The GT is a great looking car and actually felt quicker although official figueres suggest otherwise (who knows
). It felt a lot like my 147, well more than the 159 did, just a bit quicker. The problem was the fact that the interior was identical to my 147, and although it was a nice place to sit, I thought I would feel a bit bored having had my 147 for 3 years then having the same interior for a further 2 years.
I then drove the 159 and at first wasn't convinced. I was 29 at the time and the 159 felt like a "grown up's" car. 4 doors and a boot seemed to scream family guy to me! It took a couple of drives in each car and a lot of deliberation before deciding to go for the 159, mainly due to the new technology on the car. Blue&Me, safety features and practicality all shone through. The 159 was just a better all rounder for my job, I have to drive quite a bit often carrying equipment.
What I can say is that I still love the look of the GT and would seriously consider one still if my job was different. However, with regards to the performance side of things, my 159 1.9JTDm has just had it's first service at 14.5k miles and it feels better than ever. It pulls cleanly and strongly and cruises happily and now returns between 38 and 42 mpg. It is also really impressive just how composed the car is on twisty roads, more so than my previous 147. I'm sure a lot of this is to do with the passive saftey features installed as there have been a few times when driving an unfamiliar road I have been approaching a bend which is tighter/longer than I have expected yet the 159 glides round. I'm pretty sure the 147 would have given me a fight.
Moving on to the build quality, during a few early niggles with my car I was given a white GT from Western in Edinburgh (Fraser has some pictures of it on here). The car looked the part and the 2.0 JTS was fun to drive but driving it merely confirmed to me that I had made the correct choice. There was a terrible rattle from the boot, the suspension was noisy and the cabin was just not as comfortable.
Like I said, I would still have a GT, ideally a 3.2 for weekends and away days but can honestly say that the 159 has served me well and I enjoy it probably more now than I ever have.