I have both and personally I think Virtua Tennis 3 is a little more fun. Top Spin 2 wouldn't be so bad, but when you change court sides, you are forced to play on the far court (away from the camera). I find that annoying, you can zoom in to get around that.....but still annoying.
Yeah, Virtua Tennis 3 seems a little more arcade, and Top Spin 2 a little more realistic, I don't think that difference is overly massive though.
And for online, I'm almost positive both support online, but I only play single-player, so I don't know which has the better online community or better match making/finding ability. hth