Haki of course

Good question :)

whats haki (haven't seen all of this)

Definitely Haki!

Devil fruit of course!!! I'm not a very good swimmer any way!

Got to hit those Logias! Have to say Haki!

there is no bad part or side effect about haki

haki is more powerful but a devil fruit is more fun!

Haki. Simply because there are no bad side effects and you know what you are learning with Haki. Devil's Fruits are cool as well, though. XD

Haki no weakness

^ Well, that depends on the user's strength and mastery of Haki... there are weaknesses, but less so than DFs.

It'd be a bit of a gamble, but I'd make it. A devil fruit has a high likelihood of giving me an interesting and fun power to work with that would catch people off guard. Haki is great and useful, but it's far less malleable to the situation and fairly set in what it can and can do. Of course, if I knew I was getting a useless devil fruit or something I wouldn't want (such as most zoan fruits), I'd go for haki.

Devil fruits more interesting but haki more useful and if used rigth can be more powerful too.