I had a base 501X (not Worlds) and then got the 511 and as everybody agrees there's not a lot of difference. The 511 is easier to work on (diffs as mentioned) as well ask requiring far less spacers and shims which I always found a pain to deal with trackside.
The 511 is a little more nimble, even moreso with the upgrade kit... I find it settles a little quicker and is perhaps a little easier to tune.
You need to consider availability also, 511's a quite hard to come by these days.
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