Question 2 bands on small gun

I have a single band 75cm speargun which I have used quite a lot. It came with regular 18mm hollow screw-in euro bands. Quite powerful. When they wore out I replaced them with a 3/4" / 19-20mm bulk rubber loop. That eventually wore out and this year I decided to switch to a single 16mm bulk rubber band. I think it will perform about the same but will be more balanced with the thin 6.3mm spear, making it more pleasant to use.

I bought my first double band gun recently, just to try it out. It was not a sensible purchase in some ways but I knew that, it is an experiment. Also, I know that I can easily convert it to a single band gun later and I will use my 75 and 90 single band guns most of the time. I can confirm Foxfish's assertion that it does take longer to load (also see Rob Allen's YouTube video on loading a twin band gun).

My 2 cents: Just use what you've got for a while you'll eventually figure out what makedodt sense for you. The factory configurations are usually good, balanced, get used to the feel of that before changing things.

For me, moving to a shorter lighter speargun became the obvious next step. Then getting an additional longer light speargun, for clearer conditions.

If you find you need more range a longer Speargun usually makes sense. Faster tracking? A lighter, slimmer, smaller Speargun might suit. Insufficient penetration? Then perhaps a double band or roller speargun. Trouble loading? Double bands with thinner rubbers perhaps. etc..

Get some more experience before changing things. Single band guns are usually quite powerful. If you watch YouTube spearing videos you eventually notice that quite a lot of shots would be fine with just about any Speargun.

If you encounter a lot of those big fish in reasonably clear conditions, you might consider something like a 100cm Rob Allen railguns or roller gun give or take 10cm.