Home Forums Hunting & Shooting Ask The Gunwriters High shelf, low shelf AR 15 lower?

Can anyone here enlighten me as to the difference of these 2 types of lowers? I have ordered a low shelf billet lower, and now I'm hoping it will work with my RRA Predator Pursuit upper. The lower is available at Denny's Guns, AKA Global Tactical, but it's pricey.

Thanks for your help,
Frank