For those two sleds, it would seem to me that the 4000 would not be much of a step up from what you are riding now. The MXZ will have a lot more power if the seating position fits you. Carbs do require a bit more maintenance, but when the EFI breaks, it's spendy to fix.
I'd go for more power, you can't have too much of that!
2007 RMK 700, 2008 RMK 600, 1995 AC Prowler 2-up, 1980? AC Cheetah