The smt6 has:
1 analog/frequency output (For changing a voltage, or frequency based signal. i.e. a narrowband o2 sensor / maf/afm/map sensor, or like a speed sensor)
1 driver circuit (For driving injector<s>, or a pulsed solenoid, i.e. n2o, or say a vvt oil pressure solenoid, or a boost solenoid)
1 ignition frequency hicky, for changing a few different types of ignition signals, OR driving a coil directly.
1 triggerable ouput (actually input, it's a switchable ground, triggered by various things) for tripping a relay (which can turn on/off whatever you wanted)
the smt7 has all of that, plus a second analog/frequency ouput
It can also map the analog/frequency outputs to help tuning - useful in doing a MAF to MAP swap...
more inputs
you can hook it to a wideband & have it help you figure out what changes you need to make to get the engine to do what it wants. (Or a narrow band if that'll suit what you're trying to do)
It has more inputs to scale your tuning too
A way to add more types of ignition pickups
3 triggerable outputs
That's just what I know off the top of my head. They both:
Can log all data when connected
Live tune (Blah FTC's suck! LoL! What annoying little things!)
Have switchable maps
Have more than just 2 tuning paramiters (I.E. the common RPM * TPS table - which is *very* useful once you actually start using them)
They're just neat little things.
*the* signle absolute best thing I can tell you to do. Is download the smt6 software, and download the smt7 software.
Register here and download.
http://www.perfectpower.com/downloads/login.asp
There is alot you can do with them. Especially that smt7. It's just crazy.