Karate can be great if its the right style and your in a good class which sadly many of them are not but for Karate i would suggest...
Keep in mind some strikes of Karate and so forth are illegal in MMA.
Kickboxing is very good aswell and great for MMA.
The difference is huge so i can't explain it much although Karate has many forms of standing while Kickboxing really has one and also Karate teaches many strikes(punches, elbows, kicks etc.) while Kickboixing is just kicking and punching but once again they are both great for MMA and its a hard choice.
Over all Karate is harder to learn not because its better but because theres much more to learn than in Kickboxing which is great aswell.
Which is cooler? its a very hard choice for me because they are my 2 favourite stand up arts but i suggest you go try them both out for a lesson or 2 and make your mind up...if they are both great classes then it will be a very very hard choice but be careful of mcdojo's.