The VF30 and the VF34 are the same turbo in most respects except the 34 has better bearings in it, so it kicks in a few hundrew RPM sooner. The peak hp is the same however. I've heard rumors that the 34s had some reliability issues, but I don't know if there is any truth to this. I went with the 30 because it was cheaper and I was able to pick one up used with some STi injectors for a great price.

Many tuners including TurboXS say the VF30/34 is the best "street turbo" because it gives a good horse power gain without causing too much turbo lag. There are bigger turbos that will work better numbers on the dyno, but you'll rarely get to use all that power. I think the VF34 was the turbo used on the 2.0L STis (the 2 litre STi never was sold in the US) as well, so even Subaru thought it was a good match for the size of the engine.

Cobb, Vishnu, TurboXS. They all offer VF30 upgrade paths, so their is a lot of info out there. I love mine. I don't feel I'm stressing my engine and it was very easy to get used to the new power curve. It pulls like a freight train now!