The mid gas system is easier on the bolt, extractor and carrier but, the carbine system has been around for a long time not to work. As long as the carbine is ported correctly it should run fine. If it ejects the case around 1-2 O'clock have the gas block port sleeved to around .062, that should slow the carrier speed down and move the ejection angle back to 3 O'clock or straight to the right.
If the 2 rifles were side by side and the same quality grab the mid -length.
The 6.8 is the fastest growing cartridge and #1 choice for hunting deer and hogs with an AR15.
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