I've tried both. Slimv was always very buggy for me, especially the REPL buffer. I think this was because it was made long before any of the new Vim async stuff existed, and so had to do a lot of ugly hacks to get a reasonable REPL.
Vlime was made after Neovim gave Vim a kick in the ass to add async, and it takes advantage of all of it. This lets its implementation be a lot cleaner and more stable, at least from what I've experienced.
One thing I do that makes the REPL a lot nicer: I run the actual SBCL process inside a Neovim terminal split (with rlwrap). This gives me an actual REPL like you would expect, not just Vlime's "REPL" (which is essentially two separate buffers, one for input and one for output).
Vlime was made after Neovim gave Vim a kick in the ass to add async, and it takes advantage of all of it. This lets its implementation be a lot cleaner and more stable, at least from what I've experienced.
One thing I do that makes the REPL a lot nicer: I run the actual SBCL process inside a Neovim terminal split (with rlwrap). This gives me an actual REPL like you would expect, not just Vlime's "REPL" (which is essentially two separate buffers, one for input and one for output).