So I am am about to venture into the NFA for my first time by building a MK18 ish clone, and two suppressors. The MK18 I am building is going to be a block ll clone but I am stuck on the barrel length and profile. I had some people I know say to go with a 10.5 instead of 10.3 because of reliability. Is this really the case? Or should I just stick with 10.3? Also I Am looking at either the Ballistic Advantage 10.3 Hanson, or Rosco 10.5 Bloodline. I also plan to run suppressed most of the time so 11.5 is not really ideal for me. Looking for some opinions, and other suggestions would be great. Thank you

There’s no difference between a 10.3 and a 10.5 other than slight handguard coverage difference.

An 11.5 is quantitatively different with lower port pressure, longer dwell, less muzzle pressure/blast and more muzzle velocity/powder burn.  With proper gas port size both are reliable but the 11.5 will be softer shooting and more tolerant of broader variations in ammo.

In a 10.3 vs 10.5 choice go with preferred barrel profile, surface finish, fabrication, etc. if the length doesn’t impact fit of suppressor in front of handguard.  Some BA barrels have smaller gas ports and run better suppressed.

Both of the barrels you are looking at are great barrels.  The BA Hanson line is notably better than their standard offering.  The more rigid profile lends to better accuracy.  Then the included gas block, along with the NiB coated M4 barrel extension are well worth the $ upcharge from their "modern series".  I have a 12.3" BA Hanson on one of my SBRs and it is excellent.

The Rosco Mfg stuff is new to me, but I bought one for my sons 12.5" build.....(due to out of spec issues with a standard BA offering).  This new Rosco barrel has been great so far.  Without bagging on BA......lets just say it has been a phenomenal improvement over the one we had to return.

Can't really go wrong with the Rosco barrels.
Buy it and you'll be pleasantly surprised.
And BTW, my 10.3" crane spec barrel bought from Andro corp is also an excellent barrel. (which I think is made by BA)

No reliability difference between the two, but if you're planning on building a Mk18/CQBR Block II, you want a 10.3" barrel. If you want the shoulder of the barrel even with the end of Mk18 RIS II as it would be on the real thing, it has to be 10.3"; a 10.5" will stick out too far and immediately look wrong to anyone familiar with the platform.

Beyond that, the primary concern would be getting a barrel with the proper .070" gas port size - especially if shooting suppressed. Some options with proper port sizes would be Daniel Defense, Colt, Andro Corp/BA "Crane Spec" or FN contract 10.3" barrels. I'm not 100% sure the port size on the Hanson profile barrels, but I want to say it's .067" - which would be fine for primarily suppressed (although potentially on the small side for weak .223 unsuppressed). No idea the specs on the other barrel.

Personally, I would recommend Daniel Defense if it's within your budget, due to their excellent quality, accuracy, and longevity. Their warranty and customer service are top-notch as well.

I have a 10.3” DD upper I have thousands of rounds through with zero issues. I recently acquired my YHM turbo suppressor and have been running it for a few hundred rounds, also zero issues.

10.3 v 10.5 is more about whether you’re wanting a Mk18 build or not, you won’t be getting any appreciable velocity or reliability differences.

I’ve got a BA Hanson in one of my Mk18 builds.  I’ve had zero issues with this upper.  Runs good with or without a Surefire can.  Accuracy is very good as well.  I’d buy without reservation.