I guess it depends what you can afford.
I'm not a big fan of overpowering boats. I've never put a motor at the max HP rating on any of my boats--except for my canoe, which is grossly overpowered with a 3.5 HP . . . . .
Most of my spots aren't too far away, and the difference between getting there at 20 mph instead of 30 isn't much. Reliability, durability, easy starting in the cold are all way more important than HP, IMHO.
I might prefer a new motor for lots of reasons, but the 10 HP difference isn't what would make up my mind in your situation.
I might feel different if I was making a lot of long runs in the boat.