It depends on what you want to do with the car on a regular basis. If you like to tow things, you should choose the diesel with highest towing capacity. The 1,6 petrol should work for towing too, but it can't tow as heavy trailers as the diesel. If you drive in a city on a regular basis, you should stay away from all diesels. Period. 1,2 petrol is really only for people that don't load the car to maximum capacity or drive on motorways. The 1,6 petrol is for people that drive a lot in high speeds or need to load it to its maximum capacity. But the 1,6 will use more than the 1,2 in cities.
I test drove the 1,6 diesel (120hp) because it was the only engine they had in November last year, and it was way too slow for me. The 1,2 was not available, but I knew from the technical data that it was slow too. Since I'm not a huge fan of diesel engines, I only had one choice: the 1,6 petrol. It's at least as quick as the GT diesel. 6000 km later and it (obviously) works like it should. It's not the most powerful car I've got, but it does the job on motorways. I don't know why people on this forum (mainly from UK) think that engine is so unreliable. It's a BMW engine and there's no reliability issues in the Scandinavian forums I follow.