Forza's AI are fine one on one, but struggle in a group, just as GT6's do. It's just that GT6 is set up to never allow them to be in a group, so it's less noticeable.
If you're comparing to Forza 4, you've got a smaller selection of cars, but arguably a more diverse one and with better customization options.
Grid: Autosport is the best as far as competitiveness and feeling like you're a race driver. Low car and track selection, although there's some eclectic tracks.
The Codies F1 201X games are good if you want to simulate a full F1 race weekend, obviously horrible at anything else. Worthwhile if you like F1.
Don't buy Shift or Shift 2 on console, you'll be sad. The idea is great, the implementation is not so great. Buy on PC and mod them until they don't suck so much.
Ferrari Challenge/Supercar Challenge are good if you like Ferraris, but the customisation is limited and obviously so is the car choice. Decent games though.
F1:CE was good in it's day, but I wouldn't bother now. Buy the Codies games if you want F1.
Forza Horizon is worthwhile if you want a more relaxed open world driving experience. The tuning is limited, but blasting around actual roads is quality fun.
Dirt 3 is pretty solidly enjoyable, I got a lot of hours of fun out of it. A nice balance between simulation and good fun.
Wipeout HD/Fury are still about the most fun you can have racing, if you're looking for pure racing and you're not attached to things with wheels.
If I was going to have two racing games, I'd have Forza 4 and then I'd be biting my fingers over whether I chose Horizon or Autosport.
Forza and GT have their downsides, but I find Forza to be better when I want to get a real world car and do something silly to it (rotary MX5, wheee!). The latest example is I bought a 2002 VW Bora. In GT6 it's a standard car and it looks awful, and I had to go and buy a Golf as well so I could paint it the right colour. Forza just worked and the car looked great and sounded right too. The VR6 sound is very distinctive. It's little things like that which make me lean towards Forza.
Horizon is just great fun driving around and doing stuff, and Autosport is the best racing you're going to get without going online. Either would be a great companion game for when you're not so fussed about accuracy and you just want a good time.