Honestly, pick one(s) you're going to enjoy playing. Don't buy an expensive hero thinking he/she is going to lay waste to everything. Given the power of all the various heroes, I'm 100% convinced the pricing of heroes is set by popularity and nothing else. Seriously. I main Storm right now, and she's a monster. I picked her because a) I like AoE damage and b) I dig air/electricity based powers.
Storm is a 450G hero, Iron Man is a 1450G hero. Do I think Iron Man is 3.2x as fun to play as Storm, or 3.2x as powerful? Absolutely not. The same with Deadpool and Spider-Man. Cheap heroes are fantastic fun and super strong, too.
If you're having a hard time choosing, post the heroes you are considering (either here on in the MH thread), and we'll be glad to give you our opinions on which route you should consider taking.
Agree with nicoga.
In a game where the Marvel characters are the big draw, you should only buy/get the characters you like. Not based on how much they cost.
The more expensive characters have no correlation to "better".