I know as far as sales dicussions go on this site, this is a hotly debated one, so I'm going to give you my reasons for why Halo: Reach won't sell more than Modern Warfare 2 on the 360.

1. It's not a true Halo sequel - The sales history of Halo has shown us that the best selling Halo games were the sequels, Halo 2 and 3. There's a reason why Halo Wars and Halo 3:ODST sold about half or less than those.

2. Halo seems to be losing it's steam - I could be very wrong about this, but to me, all these prequel games in the Halo universe are starting to get very tiring. And unless they make Halo 4, none of these spin-offs are going to sell more than Modern Warfare 2, and certainly not Reach.

3. Call Of Duty (Modern Warfare) is the sales king of FPS's on the 360 right now - Sorry to come off as a fanboy, but this has to be said. While I know Call Of Duty games come out every year, the success of the Modern Warfare series cannot be denied. While I'm almost 100% positive that Halo: Reach will outsell this year's Treyarch COD game, it's hard to believe that anyone can really believe that another Halo prequel game is going to outsell MW2, a game that's closing fast on Halo 3's total sales, and it's only taken a little over 3 months to do that, verses 2 1/2 years for Halo 3, and that was a true Halo sequel. Reach is not, so it won't even compare.

Post your opinions below on this matter, and discuss. I'm not trying to hate on Reach. The new, revamped engine looks great. BUT from a sales standpoint, it's hard to believe that this game is going to outsell the behemoth that is Modern Warfare 2.

Isn't Halo reach being seen as Halo 4?

I don't think it will either, MW2 is still doing crazy numbers months after release...

I don't think Halo: Reach will sell less because its a prequel though, Its probably going to do better than Halo 3.

halo reach will easily outsell it...i think

Halo Reach sells 3.4 million day one, 5.8 million first month, 13.7 million lifetime

I think Halo's popularity/sales reached it's peak with Halo 3.

wtf who said it will? are toastboy and Halo_Reach serious?

It probably won't sell as well as MW2 on the 360 but your reasons are horrible.

Looking at the sales of an RTS and an expansion that used the same engine as a 2 year old game and did not have its own multiplayer matchmaking system and saying that these two show that Halo is becoming a weaker brand is just plain wrong.

Thats like looking at the sales of GTA: Episodes of Liberty City and saying that the GTA brand is weakening because the expansion didn't sell 8 million. News just in, expansions don't sell as well as full games.

Looking at it in perspective though shows you how wrong you are about the Halo brand strength. Halo Wars is the best selling RTS on consoles of all time, ODST is probably the best selling expansion of all time (may be wrong on that one) and Halo 3 was the most played game on xbox live in 2007, 2008, 2009 and is currently the second most played game behind MW2. Does that sound like a weakening franchise to you?

You can't tell if a game has reached its peak unless the next full fledge game of the series sells less. 

Edit:

On topic, I won't say that Halo reach will outsell MW2 since it's multiplat and a huge success, but I will say it will most likely surpass Halo 3 sales.  People aren't a dumb as many people think, just because it doens't say 4 doesn't mean people will look at it as an expansion or spinoff.  Microsoft will spend huge amounts of money advertising this game and looks like Bungie is adding new things to Reach.  I think people's unborn children will know about Halo Reach by the time it releases.

It will sell 10 million and more.