Two VERY different schools. While military training may exist at Virginia Tech, it is only a part of the experience. Tech is a great school with a diverse offering of world-renowned engineering and agriculture programs. Great alumni network and close bonds from it's original founding as a military technical and agricultural college. Big campus, lots of students and things to do, but Blacksburg is COLD and dreary (way more so than Lexington). Great sports teams and lots of student groups, fraternities, clubs, and activities.
On the other hand, the Spartan life and the immersive/demanding military experience you'll find at VMI exists nowhere else. The academics are very demanding and the level of commitment required to make it through such a grueling experience must be there... to make it through VMI, YOU have to WANT it. Nothing else compares to the alumni network VMI graduates have, though... great place to be from!
Just one perspective: if either of these two schools has what you want over your current school (and you've got to really want it to make it through VMI) then I say GO FOR IT!