Considering one of these two for an SBR. One of these better than the other for any reason? Best place to purchase the winner?

They're both good and have their individual pros and cons. Comparing the full size GHM9 to the full size MPX the GHM9 is a little smaller and lighter. The MPX is locked breech vs direct blowback on the GHM9, which is really the biggest difference between them. I'd say get the GHM9 only because I'm a B&T fan but truth be told they're both good guns.

GHM9

B&T GHM9.   I love my B&T products products.  I feel the GHM has many pros over the SIG.  Having experienced both, I prefer the B&T.  The GHM is slightly lighter but points quicker and feels more svelte.  The MPX feels bulky IMHO.  As for places to buy the B&T, that would be Tar Heel State Firearms.  I have lots of experience with them and have driven multiple times to Charlotte to meet with them in person.  They have the best prices and amazing customer service, Scott and Terry are great and have treated me like family.  They can also engrave your weapon if you intend to SBR it for free if you purchase from them.  They also have the best prices for B&T products.

https://tarheelstatefirearms.com/store/index.php?route=product/product&path=17_122&product_id=451

https://tarheelstatefirearms.com/store/index.php?route=product/product&path=57_123&product_id=381

Have both, mpx without a doubt.  The difference in recoil impulse is what makes the mpx shine.  Flat shooting and quick to recover,

I have an 8 inch MPX.  It is an excellent platform.  Very light recoil.  I can't imagine the B&T being a softer shooter even with the fancy buffer, because it is direct blow back.  Everything about shooting the mpx feels very smooth to me.  That being said I want a GHM9 as well.

B&T fanatic checking in.

Own both, my opinions:

MPX:
+Delayed action, better suppressed
+Lighter bolt, less blowback (though 9mm is marginal)
+Takes a number of AR parts (safeties, grips, etc.)
-More expensive of the two
-Mags are expensive
-Charging handle by nose makes it a little gassy sometimes when shooting suppressed

Scorpion
+Significantly cheaper than MPX (const, noit necessarily quality)
+Decent aftermarket (grips, handguards, safeties)
+No gas as receiver is pretty tight
-Blowback is a little sharper recoil
_Slightly louder that the MPX (IN MY OPINION, I HAVE NO METER)

I love both.  If cost is an issue I'd suggest Scorpion (but budget for parts, especially replacing the safety).  One isn't better than the other, they're just different.

How did a Scorpion get involved?

Oh dammit, I had too many tabs open.  Free unsolicited opinion!

No experience with the B&T but others have provided some solid feedback. My experience with the MPX is nothing but positive. Always the hottest girl at the dance during range days.

Yeah... if you want poor fit and finish, floppy charging handle and LOTS of gas in your face while suppressed... I say definitely choose the MPX.

+1
I had high hopes for the MPX being locked breech and bought one.  Put in a post sample DIAS for full auto and it is gassy and louder than other 9mm SMG hosts.
I have heard that when you port the barrel on an MPX and use an integral suppressor they do very well but have no first hand experience with that.

As others mentioned the GHM9 is straight blowback so have zero interest in anything that is straight blowback.
I know the OP was asking for opinions on only those two so I won't comment on what I think is better....and just leave my comments for my direct experience with a full auto suppressed MPX.

While home for the holidays, I picked up my B&T GHM9 and the 4.5" barrel MPX "K".  Between the two, I would go MPX.  I do plan to get the GHM9 Compact as I already have the folding tailhook brace for it (my current GHM9 has the collapsible brace).  Both are excellent, and while I love the features on the MPX, the quality seems much better on the GHM9.  Actions aside and after shooting a few hundred rounds through both (unsuppressed), they were both a ton of fun, zero issues, plenty accurate, and even my son's 18 year old girlfriend didn't notice any difference in recoil.

While the GHM9 controls are pretty intuitive, I found the controls on the MPX better suited to me only because of my time on the M4.  While the charging handle on the GHM9 is reciprocating (I switched it to the left side and my thumb continually induced malfunctions ), it's not bad (when on the right side for me).  I was disappointed with the MPX charging handle; it just seems flimsy, which is my only real complaint.

There were no issues with magazines of either one, but the MPX magazines are definitely a step above.  With all the quality of the GHM9, their magazines work, but seem pretty cheap; my only real complaint.  The good news is that B&T mags are much cheaper than MPX magazines.

A better comparison to the MPX K is the GHM9 Compact, so that's next in the hopper when I get home in April.

It's a tough choice, which is why I ended up with both

ROCK6

I have both. Thanks to the B&T, I’m going to sell the MPX

Stop sucking-off the scorpion. It’s not even the gun in question but you had to be that dude..

What's a decent price for a used GHM9? Handled one o er the weekend, kind of want..

With or without a brace?

B&T guns seem to hold their value pretty good. Considering right now you can a GHM9 pistol (no brace) from Brownells for under $1200 shipped after 10% coupon, I'd say around $1000 would be a decent deal on a used one.

ghm9c

No brace, got it for under 1000 with the case and 1 mag. Pretty happy with it.

I shot GHM9 today.  I love the size of this thing.  Shot 124 HST, 124 Hydra Shok, 115 AE 250 rounds total. No malfuctions.  Very accurate with a red dot. Pain to load mags even with speed loader. Trigger reset is awesome. Short and positive.  Again, the size is just amazing and compact.  It’s recoil is closer to an AR from my experience.