Whimsicott can force a lot of switches with Substitute / Encore, making it pretty annoying. However, there is really only one viable Whimsicott set, making it very predictable and easy to play around. I've used it on offensive teams before as Encore gives free turns of set up to your sweepers, and Tailwind is amazing for slower sweepers such as Haxorus.
Sableye is probably the better of the two imo, between priority Taunt, Will-O-Wisp and Recover you can outstall a lot of things, as well as being able to cripple the physical Pokemon that like to target Sableye's weaker physical defense. It's a good mon to have on a Stall team to beat opposing Taunt users such as Jellicent and Mew, while also being a good spinblocker.
Thanks for the info. In your opinion, would Whimsicott with Trick Room be viable?