Totally biased here. Let me get some facts strait and put out a more plausible theory.
The GT series has been out
way longer than FM. It's been out since the days of the original Playstation. The PS2 was huge and the fact that most people bought it with GT3 A-Spec.
Add this to the fact most people didn't buy an xbox and didn't play FM. So technically the first FM was the second when it came out on the X360.
And the biggest reason it sold so many copies the first couple of days is everybody was waiting FOREVER for it. People were still finishing/playing FM2 when FM3 came out. No shit they rushed out to get it release day.
Unrealistic amount of grip? Do race GT cars regularly? To me GT5 feels much more understeer than FM3 feels oversteer/overgrip.
No licenses. You mean no waste of time things that piss you off so much because you raced 2 minutes to be off by 0.001 seconds. I was a big fan of GT3 and GT4, that's one thing I do not miss.
Qualifying. In FM3 if you didn't know puts you in starting position based on your car's Performance Index and by your driver level. Yes it would be nice to actually race to qualify but you don't always start last.
You can't even talk about online because one day it's this another it's that. But with xbox live, cross game chats, and the much better overall experience that the 60$ Gold account provides.... Online FM3 >> GT5. Don't even start with the blah blah it's free. You get what you pay for, most of the time online PS3 is shite. 5$ a month is nothing considering people pay more for a freaking coffee.
Let's get this cleaned up.
Damage, Car models (i.e. working clocks and available interiors), sound and AI are all without a doubt superior in FM3. I mean there might not be 1000 cars and all but every single car has a cockpit view with all features working.
I'm sorry but at the top of my list for driving game is AI. If I want a challenge I can crank up the AI and have them race against me, not just follow a line.
YouTube - Gran Turismo 5 vs. Forza 3 - AI comparison
What I can give GT5 is environment effect and overall track appearance. Much better than FM3, but If you want to compared a new game to a year old game let's compare in a year to FM4. It's easy to say this new game whips an old games ass but it's kind of funny really when a game that was released a year ago is better than a new one.
Finally things that don't matter that GT5 fan bois go on endlessly about.
Photography mode. Yes pretty photos, but is it a photography game or a racing game. FM3 has a basic photo mode to share pics with friends but excels in car tuning/modding and is light years ahead in making custom paint jobs.
YouTube - Forza Motorsport 3 - James Elterman Customization and Livery Editor Doc | HD
YouTube - Takata Time Attack 2010 Livery Contest in Forza Motorsport 3
By no means I'm I saying GT5 is a bad game I'm just saying this once GT fanboi turned FM fanboi because it's simply better. I'd revert to being a GT fanboi if there was something worthwhile to be fanboi about.