Both would score 20+ goals if they would join United. If you're asking who you'd want in your team, it could be either one depending on the criteria you set out. If you're asking who the better player/striker is right now: Benzema without a shadow of a doubt.
Personally, I'd take Kane. 27 is still relatively young but it could be over in a couple of seasons, and you'd be in a similar position like you are now with RvP: a striker on huge wages with little sell-on value. Whereas with Kane, you could have yourself a quality striker up front for a decade if things go smooth. He's more than worth the gamble at this point.
Personally, I'd take Kane. 27 is still relatively young but it could be over in a couple of seasons, and you'd be in a similar position like you are now with RvP: a striker on huge wages with little sell-on value. Whereas with Kane, you could have yourself a quality striker up front for a decade if things go smooth. He's more than worth the gamble at this point.