16V/16VT - Used the Lampredi designed 16V engine as used in the Lancia Integrale. 142BHP N/A, 195BHP Turbo.
20V/20VT - All new modular engine designed by Fiat in 1996. 147/155BHP N/A, 220BHP Turbo. Distinctive Quattro 5-pot warble, 30MPG if driven carefully.
16VT has a slightly Different Dash, cloth interior has a different design that goes green in the sun.
20VT has Brembo brakes, the 16VT doesnt.
16VT has a different design of alloys compared to the standard 20VT's 4-spokes. Both 15in (N/A) and 16in (TURBO).
Both turbo variants had VISCODRIVE Viscous coupling (LSD) that works very well.
Tuning wise, it's now easier to tune the 20VT and cheaper than the 16VT, in the UK at least.
Performance:
16vt 0-60 around 7 seconds (maybe a smidge faster)
20vt 0-60 6-6.5 seconds (it varies car to car)
Top speed:
16vt 140-odd (I can't remember)
20vt 155
And there's a few little differences here and there. It's mainly the engine, performance, sound and the brembo's. I've owned one 16VT and two 20VT's, I much prefer the 20VT, but as ever with cars it's personal taste. I find the 20VT more modern, better sounding and I prefer the design of the dash.
One thing that's being noticed that's in the 16V's favour is that the 20VT's are rotting worse than the 16V
The cars are at an age now where high mileage, older cars are starting to show signs of the dreaded tin worm (the oldest 20V's are now 14 years old) 16v's aren't suffering so badly. One theory is that the gauge of steel used in the 16v's was thicker than the subsequent models. If buying any coupé I'd be checking the sills for rust.
Oh, there are also two run-out models for the 20V, the LE (Limited Edition) and the Plus. Highlights are a 6-speed gearbox, Sparco strut brace, factory fit side skirts and lips, different Alloy wheel design, Push-button start and a few cosmetic changes.
I think the 16VT is in a lower insurance group than the 20VT, which sits in group 18/19. I can only comment on the reliability of my 16VT, which was that it was very thirsty and was nothing but problems. I find the 20VT engine, with it's more modern design, far more reliable, if looked after. Ideally it needs to be serviced at specialists (FCSS, Torque Italia, Motor Mech, Power Fiat, Solo Italia) and not at Fiat, who haven't got the first clue about these cars most of the time!