I use both 50# gelspun and 30# dacron.
I am starting to believe that the gelspun is superior on well designed reels like the HD for a few reasons:
- You can practically measure the capacity in miles instead of yards, albeit more expensive it's a one-time cost
- HD's are among the most palm friendly reels made meaning little risk of abrasion on the hands from gelspun cutting the hands when ripping out of the reel
- Higher strength, lower diameter
- Material composition resists mildew, mold and discoloration
Dacron is cheaper, hand friendlier and easier to tie knots with.
BTW I like the uni as well and double over the loop on the arbor before pulling tight. For smaller reels I use the standard arbor knot, also doubling the loop over the arbor.