Hm good aesthetics question! my latest batch of cards has the www, but you're right - the address without the www is shorter and might be cleaner to use since business cards have such limited space.
Either way looks fine (gosh, aren't I helpful?) I'm wondering if people would get concerned if they saw a web addy without the www since www's are everywhere?
Nope, I'm sticking to my original answer - either way looks good, but without the www looks cleaner & would save you a tiny bit of card space.
Good question, good luck!