Sorry mate, I just saw your question. There's a few brands at my local hardware store and I've experimented with them all. I now avoid the "Saxon" or "Ozito" brand weed wacker lines. I don't remember the name of the one I have in bulk at the moment but it was the cheaper/ generic option and was not too soft/ not too stiff. There was also a commercial line available but it was 2-3x the price and although it worked well, I didn't think there was any advantage to justify the price jump. Since I'm in Australia, the brand names will mean nothing to you but my advice would be to head to Lowes or Home Depot, pick out a few of the lines and have a feel of them. Try them out over time (do some strength testing if you like) and you'll figure out what works well. Look at their longevity over the few months you'l have them on your guns etc. You might be pleasantly surprised how well the cheaper options work out.
If it ends up being cheaper than the spearfishing mono readily available, score! If not, you know what to avoid. Win, win