May seem like a trivial question but I'm new to building and slowly (in today's market) trying to acquire the pieces for a build.

I'm considering a mono upper and wanted to get people's opinions on the length of the hand guards.
I probably wouldn't go any shorter than a 14.5" barrel with a fixed flash hider, so I'm wondering what length hand guard to go with (especially if I use the mono Upper)...

Carbine
Mid Length
Ext Mid Length
Rifle

...and what are the benefits of each?