Hi, pretty much registered just to reply to this post
. The Mtron VST is a lot of fun, however some of the samples aren't as high quality as one would like. I think the Mtrons strength is really the variety of sounds you get more than anything else, which is still considerable! The Mtron is really a good tool at the end of the day.
However, considering that the Mellotron is really just an old sample-based instrument, have you considered just getting a sample bank as opposed to a standalone instrument? Michael Pinder Presents Mellotron is the one I run through Kontakt and it's sounds are really great, very live sounding and nice to record with, so make sure you look at that.
Either way though, the sounds on both are really good, I just prefer the Michael Pinder samples. Good luck!