I've built rods with both. The main difference I can see is the U15 has a formed locking band and an o-ring between the lock rings. The HG has a round locking band with a cutout for the reel foot, delrin inserts in the lock rings and the hook keeper slots. Both seats hold the several reels I have used with equal and excellent snugness. I prefer the HG and it's less money.

Juan