A teensy 3.1 ought to make a great Forth system. Single address space, 64k of RAM (plus lots of flash), and probably really fast.
But... this is where the "support" issue comes in. With AMForth, there's a team of people (well, at least one person, plus a significant number of users) who is actively working on making sure all of the "annoying" bits of the AVR architecture are adequately dealt with. For the more capable processors, what you usually get is someone saying "I built C-forth for this chip and it seems to work. Here's the makefile diff", and that's it. No "support", no "community."