I own both VX Ace and MV; each has its own advantages and disadvantages:
- MV has no hidden classes while VX Ace does, this means that in MV you can edit anything you want with a lot of freedom, in VX Ace you have to overwrite documented methods and you are not allowed to edit exactly everything;
- Now that RGD came out both engines can handle almost anything you want when it comes to graphics (RGD adds DirectX compatibility to VX Ace);
- MV engine is much slower than VX Ace engine (not the game itself, just the engine);
- MV uses a larger grid with 48x48 squares while VX Ace uses a grid with 32x32 squares;
- The maximum number of squares allowed in a single map is slightly different from MV to VX Ace, this makes VX Ace maps potentially bigger than MV maps even considering that MV squares are bigger (although I am quite sure you can handle everything with smaller maps);
- MV already has built-in functionalities to export to Android and MacOS (VX Ace does not);
- MV uses Javascript while VX Ace uses ruby.
There are other differences, but almost anything else can be modified with scripts/plug-ins to add missing functionalities. In the end I decided to stick to VX Ace even after buying MV. The main reason is the 3rd one: I prefer if my engine runs smoothly. If you consider how laggy RM games are on mobiles (I have read of many people having issues with maps with 10 or more events), I think deploying on PC is still the best option, and that is why I do not need the built-in export functionality that MV has to offer, but that only applies to me.
Besides those things (and some I might have missed) the two engines behave almost in the same way. It is up to you to decide which one you want, but, just in case you do not know, remember that VX Ace is considered legacy and is no longer receiving any support from the company. The only support you can get for VX Ace is from users on the various forums. Keep this in mind when deciding which engine you have to pick.
Truth be told, even RMXP is a good engine. It allows you to customize everything freely and many people already wrote a lot of scripts for it. It uses the same language as VX Ace with a slightly more limited engine. In the end I recommend trying them out and seeing for yourself which one you like the most.