Dirt 3 is very similar to Dirt 2 although they lock you out of events and cars unless you buy some DLC which pisses me off. It also entirely lacks the sense of true progressions as you have no money, don't buy cars, don't buy upgrades, don't customize anything but the horn... it's pretty lame, really. You just race, level up, and suddenly can drive new cars/tracks. That really, really disappointed me with Dirt 3. I think Dirt 2 is much better in that sense.
As for Shift 2, it's ok. The regular racing is pretty decent but just like the first Shift, the cars oversteer a ton and I found the engine sounds are really bad in 2. They toned back that "in the driver's seat" feel of the first Shift where the sounds were just blaring and it was like a rollercoaster. They also made drifting laughable. I can't believe how they expect me to enjoy "driving" with their asinine drift physics. Drive a car in a regular race - it's fine. Drive the same car in a "drift" race and suddenly turning the wheel makes the back end whip out rather than turning the front. This has a fun effect when you are against a wall (say, the outside corner wall), because the back end can't whip out so you can't get off the wall! The cars also magically dispense grease under the tires so it just slip 'n slides like nuts. I made better drifts in the regular races in an S2000 than in the RX8 Drift car during drift races.
I can't understand why they would do that. Cars that drift behave just like cars that don't drift, except for slight handling difference with oversteer and of course they have tons of power so they break loose. Drifting in GRID and Dirt 1/2/3 is a thousand times better. Drifting in the gymkhana tracks in Dirt 3 is a lot of fun, it's awesome. Those guys at EA/Slightly Mad Studios need to figure out how physics work. I mean, they even tried advertising the Shift series as "appealing to the sim racing fans" as opposed to the arcady style of earlier NFS games.
The one thing Shift 2 does well, which NFS has usually done well, is the customizing. You can tune cars up to the nines. I just wish they behaved more realistically. GT5 I thought was awesome although near the end of the career it became quite tedious. I really just think that GRID is probably the best racing game to come out in the last decade. The driving is realistic enough, there's plenty of cars and races, there's a definite feel of progressions as you work from your Mustang Boss to maybe a Skyline or Viper, and eventually you're in GT3 cars, and when you're really far in the game you've got LMP1 super cars to rip around in.