When i paid for my TR i considered both the 737 and the A320. When i looked at doing this (Jan 06) i came to the following conclusion. The main operators of the A320 at that time were the likes of Easyjet, Monarch, First Chioce, BMed etc and i did not feel that any of those airlines were going to take a low hours guy straight onto the jet. The 737 operators were Jet2, BMIBaby, Astreus and FlyGlobespan. These had a proven history of taking low hour pilots, and so for me the decision was easy, and sure enough i ended up flying for one of these operators.
In saying this, it's quite fluid, and you do have to look at what is going on. 6 months later things were quite different. A 757 rating would have been the one to go for, as both Globespan and Jet2 were struggling to find type rated pilots. You really do have to do your research, before you commit with a large amount of cash.
I am sure where you complete a SSTR has a bearing on it too. I did mine at GECAT and in the 2 months i was there, they were completing selection for major airlines and low houred guys were landing jobs every week. They dont guarantee jobs, but they do AIM to place all pilots they train. Whilst i was there 2 guys doing an A320 rating were taken on straight to an airline. The airline literally just snapped them up. They called GECAT and asked for 2 pilots, and that was that. I had to wait 6 months for a position, but all 4 of us on my SSTR course went to the same airline.
As for a risk, well i think this sums it up. During my airline interview the Chief Pilot interviewing me asked how much i had paid for my type rating. After i told him his reply was "You should have done it with us. You could have saved yourself 5 grand", to which i replied "If i did not have the type rating i would not be sitting here". He thought for a couple of seconds and then said "That's a very good point".