Haven't done much the last couple days to the car but tore apart the transmission to show the comparison of the 5spd 02M(180hp TT Quattro) vs. 6spd 02M
The 5spd version has noticeably larger teeth which would obviously make the gears stronger. If you look close in the pic you can see that but is more apparent in person.
One thing I honestly never noticed until now(5spd 02M boxes are fairly rare) is that Audi/Vw probably only saved themselves about $100 by not making it a 6spd. The input shaft has 6th gear on it (red arrow in picture below) and only the pinion shaft gear is gone, replaced by a brass sleeve(yellow arrow in picture below). I found this quite odd.
As seen in the list below the 5spd has a nicer spread on 3rd and 4th gears.
02M DXW 5spd 180hp TT
1st gear = 3.818 2nd gear = 2.105 3rd gear = 1.345 4th gear = 0.972 5th gear = 0.970 R/P 1st-4th = 4.200 R/P 5th-rev = 3.316
02M/Y GQV 6spd R32
1st gear = 3.357 2nd gear = 2.087 3rd gear = 1.469 4th gear = 1.088 5th gear = 1.108 6th gear = 0.912 R/P =
3.273(only the lower is listed in bentley?!?)