Not sure if this is an actual battle, which means it wouldn't matter who is the better killer or fighter, but which character appeals more. Nothing about being mainstream, but rather personal taste. I like both, although I prefer most aspects of Spiderman to Spawn, not because he is "main stream", but just because the art, characterisation, story lines etc appeal.
Spawn is cooler, darker , and with much more itensity than Spiderman. The thing is with Spiderman everything generally works out in the end, he's wearing a happy red and blue costume, he saves the day more often than not, and he's got no kind of problems except fighting supervillians and being depressed about having powers that he doesn't want. Spawn is probably the epitomy of bad luck, I mean being a soldier of hell is bad enough (or good depending on what you think) watching your former wife forming a relationship with your best friend and being able to do nothing. Being stalked by thousands of cryptic characters giving similiar cryptic messages, hanging out in alleyways with the denizens of the city and with this you get aa detailed portrayal of a character and the hardships he has to face having unnatural powers and living with them. Hell, Spidermans got it easy