RTP is the default assets that come with the RPG Maker engine.
if you don't have the RTP already installed, then you get the normal release, but if you DO already have the RTP (perhaps from another game, or you have RPG maker itself, etc) then you can get either one.
the RTP-free version is mostly to save space if you already have the RTP for whichever version of RPG maker is used (this one being VX Ace).