I'm looking at an order here.  Mega Arms says the 3S fits better with Standard Uppers.  
How well does it fit with a Mega Upper since the 3H is designed to mate perfectly with Mega Uppers?

I ask because I can save $20.00 buy getting the 3S and a Mega Upper separate versus a Matched set 3H and Mega Upper.
I also have 1 ArmaLite Upper and 1 DPMS Upper to use with it.

How well does the 3H Mate with other standard uppers?

The billet Mega upper is faceted. Not round like the forged. The 3S has a round buffer area that matches the forged uppers. The 3H has a faceted buffer area that matches the Mega billet upper.

Edit: If you're going with a forged upper, get the 3S. With a Mega billet upper get the 3H. The outside shape will match better.

Went with the Billet 3H Lower.
Billet Mega Upper.

And an Acu-Wedge (Ergo instead of DPMS to save a $) encase the other uppers need it.
But the set screw in the bottom should be fine.

Thank you for the details, I don't need the tail end to match perfect on the other lowers.