2K3, undoubtedly. Expands upon everything built in 2K (and thus a lot of the resources like facesets and such transfer VERY easily) and I love the feel a side-view battle gives to the game. IMO it's much more exciting to control and view a fight from the sidelines and see the actions and movements of EVERY participant, rather than seeing through the eyes of one side and never being able to see your heroes' animations, or what have you. In first person mode, you lose some excitement, I think.
Also, I find XP's database to be very confining when compared to 2K3's database. Particularly in the skill and status effect areas, and items. In 2K3 I could really tweak skills and status effects to create some really oddball combinations and give a bright, vivid variety to the abilities in the game, rather than just cookie cutter nukes and junk. There's a lot of user-unfriendly features that require you to script a lot more than enterbrain likes to advertise in XP, and frankly, if I wanted to dig into code, I would open up MS Visual C++, because quite frankly if you're going to code for the sake of customization, you may as well do it from scratch and be able to further customize things like character stat types, encounter types (maybe you want to make a game like Seiken Densetsu 3), and what have you. When I open up an RPG Maker program, it's because I want to ESCAPE code, not do what I already do most of the time anyway (also I do it for the graphics and sound resources, but you get the picture).