Freeshafting is the way to go if the vis will allow. It does have obvious drawbacks if you are hunting big structure where they could dissapear with your shaft. Shot placement is the key though. Usually if you put one in the sweet spot, even if it isn't a stone shot, you can grab him or put a second shaft in before he gets away. I think a line is dangerous, especially on big fish.
I use a 54" Spearfishing Specialties with the commercial muzzle. It's kind of an expensive upgrade for just a piece of plastic which make is a "commercial", but that muzzle definitely won't wear out on you and I seem to shoot a little better with it. This gun adapts well to carrying an extra shaft for free shafting, and you can switch between freeshaft and lineshaft quickly. you can also get shafts for biller type guns much cheaper than a Riffe or Wong.
Good luck!
When you say "sweet spot" what exactly do you mean?
I've heard various theories about where to shoot fish -- head, gill plates, eyes, etc. I usually try to shoot in the head as close to the brain as I can and from whatever angle I'm aiming -- side, over or under, but I'll shoot almost anywhere if I feel I'm not going to get another shot.