First, they don't have the ads that say something like, "Hit the streaker and win a free iPod!!!!!!!" All they have is a search bar at the top and you can change the colors to match your layout.
Secondly, Blogger's HTML template is more like Notepad. Xanga is pretty much for people who know nothing or have a basic knowledge of HTML. I like blogger because everything is all in one space, without having to scroll up and down, going to different sections like xanga. Plus, I know HTML rather well, so I like that better. (I don't use the basic Blogger skin, I make my layouts more like you see on xanga.)
And in Blogger, you can save your posts as a draft and there's even an auto-recovery feature, so in case your computer freezes most or all of your entry is saved so you don't have to start over. And the draft is great so if you want to come back and finish it later you can.
Plus, they have most, or maybe even all, of the entry features that you can get with xanga premium. There's spell check, font, etc.
They don't have blogrings or anything, but I don't mind.
One thing I don't like is that sometimes it takes FOREVER for the posts or layouts to finish updating. (Where it says "0%, 50%, 100%, you know.)
I use xanga as my public blog because practically almost everyone I know has one.