Well, there's no way to make a public definition: for which to be worse~
"What's the best; Cow or Horse?", it's all based on opinions..
But just to list up some facts:
Xml - far more noob-friendly. I'd suggest ALL newbs to start with Xml.
Sql - faster and stronger.
Tip: XML is no longer supported and there will probably not be any servers starting from 8.1 that is using XML. They have introduced an easier version of SQL, called SQLite, it is easier to start with.
The Forgotten Server has it as default, it's just to download the server and start it basically, and log in with 1/1, 2/2 or anything else. And make a character.