After a year or so without playing a racing game (one of my favorite genres), and having not played a GT game since GT2, I bought GT5 and thought it was the best racing game ever -- for about 2 weeks. By that time I had got to the end of the Extreme series in A-spec, did a couple enduro races (boring) and felt like the game was basically complete. I figured I'd finish the rest of the Special Events, licenses, and endurance races when I get my Fanatec wheel (supposedly in May). I started in on B-spec, but it got boring after the first few races and so I started playing Forza 2 while I waited for the races to finish.
...I ended up really enjoying Forza 2. I had bought it years ago for $5 and shelved it immediately, but GT5 changed my mind about it. I designed some cars I'm proud of, made a bunch of money in the auction house from them, and completed a good 50% of the career and arcade challenges before B-spec Bob finished the final lap of the Nurb 24-hour. I also learned in that time that the Xbox controller analog positions make for more comfortable driving.
When the Bonus Races started to dry up I figured I'd try Forza 3. I bought it, and haven't thought about another racing game since. The sheer volume of content and track/terrain variety is awesome. GT5 made me fall in love with cockpit view but most of the cars I drive don't have it in GT5, unlike Forza, and I can unleash my creative side when I feel the urge without blowing through cash picking the perfect colors and rims.
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TL;DR: For all the differences, better or worse, Forza has content where GT5 falls short. GT5 is near-complete, Forza is not even close.
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I like all my racing games, and I'll return to each of them when I get my wheel, but for now I still have a solid 200 hours worth of racing to do in Forza 3 just to complete the single-player and it's got all my racing needs covered while GT5 collects dust.