Overall, MV is a improved Ace.
The main advantages of MV over Ace are:
- Better native resolution.
- Built-In sideview system.
- Mac support (steam version only, for now)
- Mobile and browser deployment.
There are some other minor differences and advantages but those are the main ones.
Now the question is: are those advantages worth for you? Specially the last two since they are not available in any way on Ace. (the other two can be acheived with scripts). If the answer is 'no' then there is not much difference from MV to Ace for you.
About the plugin system you says to not understand... they are not much different from scripts, the only differences are their placements.
- With scripts you have to copy the script and paste it's code on the Maker script editor (by just placing the script on the editor, it's automatically enabled).
- With plugins, you have to download the script plugin file and place it on a folder in your game, then enable it on the Maker plugin manager.