I can only speak for Siege. . .
Mages and Thieves are amongs the most popular templates, along with tamers.
Plenty of opportunity for PvP mages and the thieves always have a good time disrupting things there.
Cons: The best PvP mages in UO are on Siege, this could be a pro to you, I guess it depends on your level of PvP experience
Pros: Good community and a guild to help you get started called NEW
Keep in mind that Siege is an all fel ruleset in all facets.