[Any] comparison...?

Programming - You can make a whole game without ever touching the RGSS in RMXP and RMVX.

Spriting - Is complicated anyway you look at it if your going at it from scratch. But the sprites are bigger in rmxp thus making them a little easier to edit.

Game Ability - Well, The bright side on RMXP is the fact that you can merge all the tilesets if you want to into one long tileset. There is no length limit, but there is a width limit. On RMVX your given the choice of the RTP and the ability to put anything on any layer as well as a 5th slot for a custom tileset. Now, The RMXP DBS isn't very good, although some people do like it.

VX has a chibi feel to it, I would go with XP.