I have accounts on all three
Hi5 is the worst. I can't stand the layout. Everythings so, I don't know. Disorganised I guess. :\ Plus you can't do much with it. And, I just find it all in all, long winded. lol
Myspace is good because you can fiddle with html and stuff to get the page looking how you want it to look. I like this the best, cause I'm useless at html, but I can easily figure out how to do it from the help on the message boards. Also, many people and bands are on this site... which is quite good.
MSN space is good for people who like blogging and stuff, but cba to get their own blog/webpage/photo album or shiz. And good for people who don't know much about html, cause it's quick, clean and simple. Made for the modern newb