I want to get a 12.5 barrel but im unclear whether the carbine legnth or mid length gas system has less recoil. Carbine cause theres more dwell time or mid length cause its farther away from the receiver?

I'd say it comes more to port size than anything. Either would be fine with the right gas system

Carbine length would be less dwell time. Bad info.
Depending on what barrel manufacturer would be more or less recoil.
5.56 doesn't really have much recoil anyway so just go with a good barrel and you should be GTG.
Here's a link to different port sizes to help with your decision.
https://docs.google.com/spreadsheets/d/1tXunBDX5Gaz87BqxwNxDUlWNK9nEv-cZEQoLq2JXXrk/edit#gid=766121382

No, carbine length is shorter than midlength so has more dwell than midlength when comparing the same barrel overall length. Dwell is the time that the gas system is pressurized, which is the time between when the bullet passes the gas port and when it leaves the muzzle.

All depends on what OP is trying to achieve.

There are numerous contributors to perception of recoil. The most significant of those in regards to gas system setup is how fast the BCG/Buffer stack is moving when it hits the back of the buffer tube along with how much mass that stack has.

It's possible to tune a rifle and ammo combination so that there is almost no impact there, and that tuning process is easier when starting from a baseline of a longer gas system, but the downside is reduction in operating envelope and overall operating reliability. There are no free lunches.

If you are looking for a "gamer" rifle configuration, that will be used with one type of ammo and malfunctions are acceptable in the grand scheme of things, then proceed to set up an overly-long gas system (midlength) with an adjustable gas block and a lightweight buffer.

If you're looking for a general purpose firearm for possible defensive use and reliability is more paramount, then you probably want a larger operating envelope, a little more energy in the operating system and a little heavier buffer to handle that energy. A 12.5 inch barrel is in-between gas systems on optimum length so you're picking one way or the other to bias the system. I have no experience with a 12.5 midlength to know whether I would trust one for a defensive application.

Having an excessive amount of energy, regardless of the gas port location, is detrimental to overall shooting feel and may pose some reliability and longevity concerns. In general it's easier to get too much energy with a shorter gas system as the pressure is higher, so they're more sensitive to changes in port size and variability in ammo pressure curves.

Dwell time is the amount of time a bullet spends between the gas port and the muzzle. 

Technically, "dwell time" is the time from primer ignition to the initiation of unlocking . . . .

Thanks for the replies. Would be interested to hear from someome whos shot both, but this is helpful.

Thanks for correcting my thinking.
It's not my intent to spread bad info.
Learn something new everyday.

I haven't fired a 12.5 ML but have fired 10.3, 10.5, and 11.5 C gas barrels.

In my experience, the greater the distance from the gas port to the muzzle the greater the reliability.  Interpolating this, a carbine 12.5 will be more reliable than a ML 12.5.

To lend a little "real world" credence to my "hobbyist conjecture", it is my understanding that the M4 Carbine's 14.5" barrel was selected because Colt determined the cartridge was not in obturation during bolt unlock while maintaining enough distance for proper dwell time.   I could be wrong about this. Some people say its because Colt already had foreign contracts for a 14.5 (which technically does not defeat their testing and selection of the barrel length).

What about a Rosco Manufacturing K9 12.5 with their patrol length gas system.  In between Carbine and midlength.

https://www.roscomanufacturing.com/shop/barrels/k9-barrels/k9-12-5-sage-dynamics-5-56-nato/

https://www.youtube.com/watch?v=QGicr5ZK7DI&t=846s

You're overthinking it.  For under 11.5 the standard is a carbine gas system.  Going larger?  Larger gas system.  I would strongly advise staying away from these new "patrol' length systems.

If you're going 12.5 get a mid gas.  Noveske just came out with a 11.5 with a mid gas and it is working great, especially with cans.  If you're going to put a can on you don't need the same level of dwell time as the suppressor is going to add to the pressure.

I have a 10.3 carbine, 11.5 carbine, 12.5 carbine, 14.5 carbine, 14.5 midlength, 16 midlength with A5, and an 18" rifle with A5.

Honestly, the only difference I feel between them is their weight. When it comes to feeling a difference in recoil, you're really kinda picking fly shit out of pepper when it comes to gas length. And also when it comes to adding an A5 recoil system.

There is a difference in recoil impulse, but not much. And if there is any recoil mitigation between all of them, I sure as hell can't tell.

So for me, I like to pick what's been long proven to work. Go carbine length with a 12.5