Visually, i'd go with what ZombieLoffe said - bright, noticable bubbles, without too much contrast between the background of the bubble and the border. I really don't see anything wrong with using circle bubbles though, just try make 'em more like an oval or wider, like in the Concerned comics. But this is all just personal preference.
There really isn't much advice you can give with comics other then make sure you spend the most time on it's humour/story and it's posing. Alot of a comics humour can come from the expressions characters and objects can bring about. Good luck with the comics!