Buy both. In the end you will have all UAD's plugins
Just demo both of them and choose. It realy depends on the sound you want. Both are great. IMO 88RS sounds round and warm, and the 4K sounds crisp and upfront. But when you tweak them enough, you will get them sounding close, but just with a differend sound(color) to them.
It also depends on the UAD you have and how many instances you want to use.