Forza is as much a competitor to GT as Battlefield is to ARMA

Forza 3's bundles are closer to 2 Million than 1-1.5 Million. It had a release bundle, it was bundled with ODST after a few Month of release and it had the Holiday 2010 bundle.

Despite this, Gran Turismo 5 did more than Forza 3's current lifetime (84 Weeks) of 4.83 Million units, in the space of 5 Weeks.

Forza isn't really a "competitor" to Gran Turismo just yet, despite how much Turn 10 want it to be. They're trying to bundle there way to success and it's still not working... So they turn to Kinect. If Kinect doesn't lift the Forza name to Gran Turismo heights, then there's probably no hope for it in the long run. Sure, reviewers say Forza is currently the better game (although Gran Turismo is the better racer, if you read the reviews), but in the eyes of the public... Forza is no GT.

The Camaro outsold the Mustang by close to 8000 units in 2010,  and for the first 3 months this year it outsold it as well.  Dont know about the other months

Um, yeah, pretty much this for me too.

I've played Forza 3 and GT5. It was honestly like I was playing the same game with different menus and HUDs.

Sales-wise? Sure, it competes. I'd argue it's even potentially MORE successful given how well it sells and how efficiently and quickly the games are developed. GT5 was a ridiculous waste of 6 years, not that the final product wasn't great.

As far as the games themselves, professional racing sim analysts have broken it down and it's clear that Forza doesn't really drive like a sim. It helps you out in a lot of ways automatically which the real cars would not, and thus is more fun for people who want something more arcady, while GT5 has those options all assists can be turned off and it becomes a real sim, with the most accurate physics in the genre. My one big complaint with GT5 (and it's pretty big) is I perfer to play in a realistic cockpit view, and it only has that view for 200 cars (most of which I don't care about).

While I know what you're getting at with that comment, I think you should to take the following into consideration -

Gran Turismo 4 - 2004/2005 - 11 Million Seller
Tourist Trophy - 2006 - We only have Japan sales here at over 100k, but the game went Platinum so it's safe to say the game sold over 500k WW.
GT HD - 2006 - No data
Gran Turismo 5: Prologue - 2007/2008 - 5.2 Million units (Inc Downloads)
Gran Turismo PSP - 2009 - 2.3 Million (Inc Downloads)
Gran Turismo 5 - 2010 - 6.3 Million sold in 7/8 Months (Had shipped 6.3 in 2 Months)

6 "Titles" released between 2004 and 2010, without Tourist Trophy and GT HD included here, we reach almost 25 Million units of SW (Inc DL)

And then we have Forza...

Forza Motorsport - 2005 - 1.07 Million
Forza Motorsport 2 - 2007 - 4.01 Million
Forza Motorsport 3 - 2009 - 4.83 Million

3 titles released since 2005, not yet reached 10 Million units.

Make of this what you will.

I'm more eager to compare Forza 4 to GT5.

Your never going to get peoples honest opinions when you compare titles that are exclusives to the competition so why bother. GT5 has always been better than forza and Forza was MSs answer to the GT series. But of course I'll say that. I grew up playing playstation and Gran Turismo and thats my honest opinion. These type of topics are always hilarious. You get some people saying one game has more real sounds than the other and these people never even heard what the car they picked in the game sounds like in real life but the sounds are more real than the other LMAO.

The question I have is why ask which is better. If you truely think something is better than just play it and enjoy it.

Your data is pretty skewed. First of all, scrap anything before this generation. The PS3 and the PS2 might as well not even be related, cause that audience clearly didn't transition well. In the interest of fairness that also means you can drop the first Forza, which gives us just Prologue PSP, and 5, which paints a much more even picture. Consider again that two of those GT games were not full price ($30 and $40 and the $30 one actually sold most of its copies at $20 or less), and that Forza has yet another release this year and it should easily pass it in franchise success for the generation. 

If we're doing that, then I think it's only fair to consider that GT5 Prologue is... Well... Pretty much a demo, and GT PSP was released on a dying platform, absolutely owned by piracy. Add in the fact that a huge chunk of GT4's sales came actually during this generation, but I'll let you off with that last one

Even then... GT5P, GTPSP and GT5 all combined are still at 13.8 Million, should be pushing close to 15 Million by the end of the Year, maybe more, if we have GT5 undertracked here (As it looks).

Forza 2 and Forza 3 are sitting at 8.84 Million currently. Hard to say where it will be by Years end, as we don't know what bundles they will be giving out, if they do infact bundle the game. I think it's fair to say that the Forza franchise though will be somewhere around 11/12 Million by the end of the Year. And that's being generous really, as it took F3 a full year to hit 3 Million (With a launch and ODST bundle), I'm giving F4 3 Months to do it! (Forza 3 did 2 Million in 3 Months) It will still be far behind GT for the gen, and it'll be hard to catch up...

I think GT will end up with higher sales for the gen, despite only actually releasing one full game on a home console. And if GT6 is a PS3 game, Forza has no chance.