Both will fit your Spikes lower. Get the milspec diameter, life will be easier if you start swapping stocks down the road.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek
Mil spec is the way to go. Stronger tube, easier to find, dunno why they even bother with commercial tubes anymore.........
__________________ Assumption is the mother of all ****ups. Know what you know, Know what you don't know. -Paul Pedzolt "Jack of all trades, master of none, though oftentimes better than master of one" -Mynshul