I prefer Monster much much more over 20th Century Boys.
For one, Monster is more realistic. Also, the theme itself for Monster - the monster within human nature - is, well, more intelligent and meaningful?

Sometimes, I get this feeling that 20th CB is a little pointless... =|
What is the point that the author is trying to get across?
That rock music has great moving powers??? *scratches head*

But nonetheless, it has a complex and intriguing plot.
Looking forward to the release of Vol 21. Hopefully it'll finally bring an end to this Tomodachi saga.