Everyone has different tastes, even the obvious advantages/disadvantages couldn't win against one's own subjective. Here I list down everything I love and hate in XP, and what VX/Ace has, maybe can help you decide :
1. I love XP tilesets and charsets, I hate VX/Ace dwarfed tilesets and charsets (This is the main reason Why I'd go with XP. I wanna play RPG, not bomberman
)
2. XP is using obsolete, slow engine while VX/Ace has newer, faster engine. VX/Ace has more features while many scripters claims that VX/Ace has dropped several must-have features. To take a note here :
Features can be achieved by using custom scripts. And here, has the biggest XP script resources database. An example, the RMX-OS introduces online multiplayer capability. Who ever thought playing RPG Maker games online was possible.
3. For serious mappers, VX/VXAce has major disadvantage (my opinion).
A) Automatic Layers in VX/Ace = serious mappers hates them while casual mappers loves their easiness.
B) I Like VX/Ace shadowing tool. I was hoping someone can make map shadowing script for XP
C) VERY limited creativity while mapping VX/Ace Map. Not advertising, just give you an example of one good XP mapping :
http://forum.chaos-project.com/index.php/topic,11546.0.html, that is XP mapping looks like. In short, XP graphics by nature has successfully deliver the atmosphere; VX/Ace = sandbox map, I can't feel the "map", my humble opinion despite them having better color quality and depth.
I'm not XP fanboy, prior from using XP for more than 4 months until now, I've been using VX for more than 6 months, only for fun. Gave me basic understanding of making games using RPG Maker. In the end, after many consideration, I chose XP. Dunno this help, just share my thoughts