The difference between Isak and Toney is that Isak is 22 and had a good season last year, though his forms dropped a bit this season. Toney is 26 and is having his first top flight season and doing ok. I'm more wary of Isak after this season but you are still buying potential. Toney, DCL, etc are what they are, they're a lot less likely to go up a level.