Have owned both and imho the difference in performance is negligible. Even compared to XT there's no fundamental difference in stopping power
The only thing I would say is that good as the Deores are, they have a workmanlike look-and-feel to them which may or may not matter to you. In the usual way, the SLXs go some way towards improving on this but it's not until you get to XT and the rarefied atmosphere of XTR than it becomes clear how much of a bargain-basement brake the Deores are. But if you can live with that, functionally they are great brakes - just don't look at them too closely!