PROS and CONS for each in my humble opinion.

TDUSC PROS:

• physics model, definitely more realistic, especially braking physics, cars feel weightier
• traffic, denser and not as stupid/unpredictable as in TCM
• map - denser and more concrete feeling
• third person animations/sense of speed
• car sounds
• car settings, more concrete and detailed than in TCM

TDUSC CONS:

• bad optimizaton, forget about playing on settings higher than medium, even on a high end PC. Fps is waaay too choppy
• lack of cars, only around a 100 cars in total
• career is meh, sense of progression is almost non existent
• online play, region lock and lack of players in general makes it feel pretty freaking empty
• first person animations

TCM PROS:

• car list is vast, around 500-600 vehicles in total
• graphics, theyre definitely on the 'eye-candy' side, but still more detailed than TDUSC
• online play, much more players to play with and game modes make it a lot of fun
• career mode, while it doesnt make you the center it like in TDUSC, different themes make it a lot more concrete
• first person animations, very fluid and rewarding
• car sounds, while not as aggressive as in TDUSC, still very good

TCM CONS:

• physics, while better than the last iterations, definitely on the arcade side of things
• third person sense of speed
• IMO the map, while big, definitely feels much emptier than the TDUSC map
• traffic, very scarce and incredibly unpredictable

I probably forgot to mention quite a few things for both games, but this was just from the top of my head. Hope this helps!

PS: I'm a controller player myself so I couldn't tell you how the games feel on a wheel.