I know that they are pretty different, but what is your opinion?

Since jungle is the only position Mundo is comfortable in, I assume that's what you're looking at for this comparison. In that case, Mundo is the stronger jungler in my opinion.

Gang plank has such a fun kit, his q letting him farm and harass like a boss his w making cc non existent and his r letting him stop pushes and kill people no matter where he is. Unfortunately he's weak, but fun none the less

Hmmm do I want to go where I please? Or be a pirate?

Both incredibly intriguing options.

Both have super fun poking abilities (GP popping his pistol in the enemy's face, and Mundo throwing a cleaver at people)

Mundo doesn't die when played well which is just hilarious when you get out of teamfights that normal champs should have died in.

BUT

GP has super awesome Kill St...Secure Potential With his Ult. Which is hilarious because you can be wrecking face without even having to really do work.

Hmm....

For strictly fun...

I'd have to say Mundo (note: none of this is taking into account actual gameplay and balance, they're both pretty cool champs to play)

because he goes where he please:

http://youtu.be/-bYJiC1V9Hc