i have both. longboards are way faster and more fun to cruise on, but you don't have much agility (impossible to make sharp turns and fast stops). they also let you ride over sketchier surfaces like patches of gravel and grass and big cracks in the sidewalk. wicked fun for hills too.
skateboards are way more versatile. you don't have to pick them up when you need to get on a sidewalk, for example. you can jump over stuff, learn new tricks, and ride off of stuff. i'd say it allows you to do more things if you're interested in pursuing the tricky aspect. if you're not, you can still enjoy pivoting and tic-tacs.
if you want to be able to cruise around pretty well but still be able to ollie and kickflip, get a cruiser deck like the zig zag that was mentioned by someone up above