Another thing to look for besides technical differences (ie the things Winterfire mentioned)
is I would look into weather or not the two programs can use the same art assets, and if not, then art asset availability may influence your decision.
I don't use RPGM nor have I looked into it much, but I do like to look into tile sets. Maybe (and this would need to be checked/confirmed/or denied) the two programs use two different size tile sets, maybe one uses a 32x32 size tile set, but the newer one might use only 64x64. In this case, just like the older one has more plug ins and more tutorials, it may have more usable art assets to explore. The newer one may have higher quality assets you can get, but you may be limited in your options.
similarly, for 3D models I went with Daz Genesis 8 because while it was still new all the artist I liked and that made content for it wore switching from G3 to G8 so I went with G8 just beacuse I wanted asset support, and after a year it had more assets than G8 (on account that Daz the program became more popular), and now that G9 is out G8 still has more art assets and will continue to have more for quite some time.