It just depends what you plan to do.
I've had hunter friends (long time hunters) state they've caught just as much stuff using a gig (hawaiian sling syle) as they did with their guns over the years. It just depends what you're going after. If you're shooting big game fish... you might want that extra power of the speargun, and something to hold onto if you hit it. If you're shooting smaller stuff, a pole may work fine and be all you need/want.
Different tools for different jobs and mostly just personal preference.
I have one of each.
Range: Depends how long your line on the spear is, and how powerful the bands you use are.
Maintenance: just like anything else you take in salt water, good rinse, grease/oil, etc. You will need to periodically replace the bands on either one though. The spear has one band, the guns have one or more.
Try them both, may shops will rent that kind of stuff, or borrow one.