Hello, I have an 18” 556 upper with a x15 lower. Should I go rifle or carbine buffer length? Upper was prebuilt brand new from aero

Finally snagged me an Aero Ambi Pro Lower to go your with M4E1 upper and Pro handguard. OCD is strong with me. Moved my UIC lower back to it's upper.

Prefer the smaller profile logo by trigger pins.