Where are the differences ?
Is it only in the turbo part ? or the engines differ as well ?
all I know from terminology: one is 10 valves, one is 20 valves.
What about in the tranny ?

And physical appearance, will the 1990 200TQ differ much from the 1991 200 TQ

20vt uses a different crankshaft, rods, pistons, and obviously the head/cams/valves. The 20v head is a MUCH better design in all respects than the 10v head.

The valves of the 20v are actually smaller in diameter, but when combined they flow a lot more air.

Turbo's are both K24's but the 20vt has a different turbine/compressor wheel combination.

Intake and exhaust manifolds are vastly different, as is the exhaust (20vt gets a dual pipe exhaust).

Intercooler of the US spec 10vt is the crappy dual-pass (inlet/outlet on same side) whereas the 20vt gets a nice single pass (as does the european spec 200 10v, see my previous posts on that subject from the past couple days).

Engine management system of the 20vt is much more modern, electronic injection vs mechanical, electronic air flow meter vs mechanical. Far greater tuning potential from the factory.

Body wise, the 200q20v got flared fenders front and rear, as well as different (wider) BBS rims.

Brakes were changed, larger front and rear.

Transmissions were both 016's equipped with a TorSen center differential.

Thats all I can think of off the top of my head.

Everything he said, except:

1986-1988 10v turbos(5000cs quattro) had a larger k26 turbo and no torsen. Torsen showed up in 1989, at least on the type 44's. In addition, 91 200q's had front ufo brakes, whereas earlier 200q's had g60's. Altogether, the 91 200q was a totally different animal from the earlier 5000csq/200q's. A good way to end the type44 era, if you will. HTH

Rokas
93 100s

200 20v has essentially the same engine as the UrS4. Air induction is a little different, and it has a distributor instead of coil pack. But the block and head are the same

Basically...200 10v is OK car...200 20v is FAST and really cool car.

Which would you rather have ?

Performance, comfort, luxury, repair/maintenance.