I stick with XP because it can do everything all the others can without having to jump through hoops.
Mapping:
XP's mapping is superior in every way IMO. Sure you can port your graphics from XP over to VX/A but that takes lots of time and lots of planning just to get it to work correctly. A different graphic per layer is a huge pain. I wanted some objects to overlap other objects in VX/A so i had to use the same object 2 times on different images just to get it to be on 2 different layers where in XP all i had to do is select a different layer saving me 5-10min.
Autotiles are a huge pain in the ass. I hate the fact that you have to know what sections of the image are suppose to show up transparent, what ones are animated or not. Same with the walls. Took me a while to figure out that the top 2 ground autotiles are solid and the bottom 2 rows are the ones that show up transparent. Sure you can use parallax mapping but you shouldn't have to jump through all those hoops to get the game to look like you want it.
IMO the mapping is the #1 thing in any game cause if you have bad mapping chances are people are only going to play the game till they see the mapping. You could have the best story and gameplay in the world but bad mapping can ruin a whole game.
Database:
Sure the database has a few nifty things like notes and a few things here and there that i dont use anyways. Not much of a difference here.
Sprites/Graphics:
Only thing i really liked was the icons. Having 1 sheet for all the icons is a good idea. Not sure if theres a limit of how big the icon sheet can be but if there is another limitation you can run into if you have lots of items.
The character graphics are not bad but annoying when you have to have a !$ in the file name in order to show them as a single character per graphic.
Now chibiness is well meh in my book. I thought we broke away from chibi characters in the 90's.
Gameplay/Scripting:
Theres lots of neat things they have done with the gameplay like sprinting, airships, boats, TP, and 60fps. But all that stuff can be done with scripts in XP. Sure 60fps is nice but the human eye cant see more than 30fps anyways so i dont see why people are always "But its got 60fps and that makes it worth it and better"
The screen resolution is something every RPG Maker was bad at. Give us at least 720p(1280x720) or custom resolution enterbrain instead of this 90's 640x480. I highly doubt anyone has anything lower than 720p now days.
RGSS3: I havent played with it to much but with what i have played with i still prefer RGSS1.
RGSS3 to me in a nutshell is methods inside methods inside methods. I cant remember what script i made an edit to but i had to edit 4 scripts to make 1 little edit.
Events: Havent noticed anything new/worth using than XP.
Final Notes:
Give me XP's mapping, sprites, and icons and i would switch over any day.
So thats all the reasons im still sticking with XP. Mapping being the main reason cause the tiles are a pain in the ass and still limited like crazy. IMO VX and ACE are a step backwards from XP.
Sorry for the little rant. People need to know where some veteran RPG Maker users stand.