As already mentioned, Which? has done many a test on these things and reading the report taught me a lot. British standards testing not including side impacts for a start. Which? testing standards do.
The results suggest that isofix "generally" performs better but that is not to say that some belt mounted seats aren't better than some isofix models.
I'm not saying that Which are perfect in the way they carry out their testing regimes but they do provide some sound scientific basis upon which to base your decision. IMO isofix is way quicker to swap between cars and they can be belt mounted if you don't have the fixing points. I have one of each (didn't have isofix at the time of first sprog) and prefer the isofix for its solidity and ease of fitting (Britax like others on this thread). Having said that, the Maxi Cosi Tobi looks more comfortable and feels fairly solid for a belt mounted option (performed well in the Which? study).
From a safety perspective just yank the belt mounted models sideways (and imagine the door traveling in the opposite direction) for a comparison on side impact performance. Side padding also important as amazingly some seats have very little of this.