I think a lot of it depends on your goals....there are several posts out there on this very topic.
The 51 seems to be on the fringe in terms of needing stall or not needing stall, and it's a very subjective discussion.
I had a TA51 which I had upgraded to directly from the stocker. Now, I had a puck alignment issue which hurt my spool a bit, but nonetheless it seemed that it probably would have been happier with a bit of stall - 2800 to 3000. I originally went with the 51 because of the top end potential of the turbo as opposed to the 49, and I thought that I'd be able to dial down the boost a bit with the 51 and still achieve similar results.
I swapped it for a 49 because I determined that it was better for my goals - a street driven/show car with no track time. Plus, I was reasonably sure that I was going to end up wanting a little more stall. Therefore, I think the consensus is that if your car is largely a street machine, the 49 is probably one of the best all around turbos, and its very happy with the stock D5. The 51 moves into that area where you start needing a bit of stall unless you're happy with a laggy first gear.....
Just my .02
Rich