I have both the spincast and underspin for UL fishing.  Fun reels but just wanted some opinion which line you use if you own either reel and what pound test.

I am not sure what your typical fishing day looks like (what type of water/cover you fish, what species you target, and what types of baits/lures you use), but I feel that 4-6 lb mono would be a good "all around" choice for most situations with your particular setup. Mono may not be as sensitive, but in my opinion, is certainly more versatile for lightweight rigs.

I wonder if ultralight braid, because of its small diameter, would dig into itself from time to time and cause a nasty snarl on your enclosed spool. I also think about fighting fish. Regardless of line, its easy enough to bully bluegill, crappie, and bullheads out of the water without playing them much--- but to hook onto something bigger, I would prefer a line that is somewhat forgiving. Mono has some stretch to it, so I feel that it can absorb the impact if a larger fish makes a sudden run or takes to the sky. In heavier applications, braid could pretty commandingly horse a larger fish into your landing net, but on ultralight line and tackle, you are at higher risk for a thrown hook, tear-off, or snapped line if you don't bring your A-game.

Did you purchase these reels as part of paired rod/reel combos? If so, it might be helpful to look and see if the manufacturer of the rod printed a suggested line weight on the blank between the reel seat and the first line guide. People don't always follow those suggestions, especially if they have a very specific use in mind for the rod/reel, but I have found that it is a very useful place to start.

I have the underspin for my grandson and I have it spooled with 4lb mono. I have to say that it performs so well I really am thinking of getting one for me crappie set up, very smooth drag and for 2 full years it never tangled, messed a cast up or had any regular issues associated with a spin cast reel. 

The pin the winds the line onto the spool on a spincast reel will NOT handle the braided line.

I have both the spincast and underspin for UL fishing.  Fun reels but just wanted some opinion which line you use if you own either reel and what pound test.

 

I actually used 10 pound Spiderwire back in the days we had some of these reels. Worked fine for the most part. Can't remember any problems.

 

We also have some Abu 276U underspin reels that came with 8lb Trilene XL. Not UL size, but worked well, landed me several large pickerel, one being ~24+" long. I did try 2lb test on one of the small Zebcos, not a big fan of 2lb mono, but that's me.