I'd look at the youtube videos for each game to get an idea of what the gameplay is like. I don't own any consoles, so I don't know what Midnight Club is like. Looking at the videos, Undercover races are more track like in that the streets have more variety of turns.
NFS Undercover isn't the greatest NFS ever made, but it's reasonable. The storyline isn't great, but it is a racing game as opposed to a movie (many of which are worse than Undercover).
Looking at the videos, the graphics seem similar. Undercover defaults to an excessively bloomy and blurry look on the PC, but this can be turned down or off. With the blur effect turned down, the graphics look very nice.
The fastest cars are a bit too fast in my opinion, too much power and grip. I describe them as the equivalent of Star Wars swoop racers instead of cars. This started with ProStreet and continues with Undercover. However you don't have to run these cars offline, and for online racing, the game auto-switches between levels of cars.
Anyway, the racing modes include point to point events, such as checkpoints or sprints, and circuit tracks. The pursuit events are similar, with the only difference being a goal to take out so many cop cars or cost to state (best done by taking out cop cars so it's the same thing), and simply escaping without any damage to the cop cars requirement. Some "jobs" are takeout events where you crash into other cars to disable them. Other jobs are steal a car, evade the police, and deliver the car. Outruns are similar to the previous games, where you get a lead on an AI car and then maintain the lead for some length of time and you can drive anywhere. Highway battles are similar except the goal is to get a 1000 foot lead in some period of time, you're stuck on a freeway going in one direction, and traffic is heavy; at the higher levels, it's mostly sheer luck to just survive a run between random patterns of cars.
Still overall, it's a descent game, with 100 miles of roads to play with.