There's a reason they are both within my favourite players list, as I see it, technically speaking, Buckethead can shred Slash away, but that doesn't mean he's a better guitar player. Slash has his unique way of playing and expressing himself through the guitar, and so has Bucket in a way his Soothsayer is a guitar orgasm for real
It really comes down to a personal preference like everyone, but my vote goes with Slash