Well,
The choice actually depends on what type of fishing you are after, really, or even what type of waters you are fishing...
My personal options are based on the following:
A good all rounder for european waters (Atlantic) is the 90 cm, eurostyle, single band musket (I voted for that). It has sufficient length to be precise in shooting long-distance (think bass here), and not really very cumbersome to move around. Under very low vis, I switch to a 75.
I never use double bands. that is because it has been shown that although they tend to lend some power to the initial thrust, one looses in med- to long distance accuracy and the distance fetched by every shot is not really improved. Another downside is that it increases resistance to lateral moves, as when you are trying to follow trrough a fish going by at a perpendicular to you - lots of resistance added and a freaky vibration that scares the crap out of decent bass. There is another more dangerous aspect - with two wires stuck on top of eatch other at the spear notches when you are loading, bad things are more likely to happen - read serious injury to your fingers when one of them gets loose. Plus, looking badass out of the water is OK (I guess) but the fish will not appreciate that a lot, and I tend to favor the stealth approach. For more distance power, I switch bands from 16 mm to 20mm diameter. For more nervous impact power, i tend to get rapid, nervous slings - the red ones, but that is not very often the case. For more impact power (read > 10 Kg conguer eels or other similar fighters), I tend to switch to a second 90 cm, with 20 mm bands and a thicker, heavier spearshaft (7.5 mm diameter instead of 5.5).
For clear waters, I use a 110 cm euro style with soft single rubbers, the latex variety. Shots are fluid, and there is enough momentum to carry the spearshaft far with accuracy. On my old neck of the woods, however, bass seemed to already know the safe distance according to length of speargun, sadly and you had to be clever
When coming out of the water, badass spirit is always shown by handling a really big fork and digging in with gusto :t large bottles of ice cold beer and a way with the grill also seem to help....