IMHO SSD5 is the best value in the top tier (sound-wise) by a mile. The 5.5 update dropped about a month ago after well over a year's delay (UI improvements, 1000 new patterns etc.), and makes it even more worth it.
The expansion pack rabbit hole is non-existent too, whereas BFD and SD offer many expensive options. Slate only has 4 IIRC, and they can be had for 50 bucks each or even less on-sale ($30 is the lowest I've seen them IIRC).
One of the great things about v5 is it offers less-processed samples for those who want to go that way, whereas in the past they all had uber-quality outboard EQ and compression baked-in. Those options are still there 'cause the library always includes all the previously-available factory ones, a philosophy I like.
That's all I've got, mate; sorry I can't speak authoritatively on the matter 'cause I haven't actually run it yet. Bought v4 and then v5 'though, and I consider myself set for acoustic drums when the time comes.