I bought a C5050 when they first came out and I still use it. I've looked into getting a DSLR, but the cost is just too high right now and selling off my camera would only gain me a fraction of what I paid for it.
Still, it is a great little camera that can do a lot- but there is that shutter lag to deal with.:head I usually set mine to HQ, not RAW, and set it to rapid fire mode- so that something turn out in the middle.rofl If I am shooting above water I reset the thing and go shot by shot. Shooting in RAW mode takes a LONG time and is best for still subjects if using the 5050- one reason why I'd like to move up to a DSLR is to illiminate this issue.
I also have the INON wide angle lens, and Dome port, which make a HUGE difference in what I am able to do. An add-on lens will get you closer and that's what will make your shots better- reducing the amount of water between your subject and the lens.
Seeing as how the 5050 was considered the standard for P/S digicams a couple of years ago I would say it's still a safe bet to start out with now. The PT015 housing is fine for most freediving, and scuba, but Ikelite makes a beter housing if you want to go deeper with it. The PT015 is only rated to 40 meters, but I've bumped into people who've taken them to 70 meters- not reccomended but possible. I used to do the same thing with my Motormarine II's- they were rated to 50 meteres and I took them to 70 meters without too much of a problem, although the auto advance is a bit slower at depth.
I don't know too much about the Sp350, but I would suggest looking over at Wetpixel, Digideep, and Steve's Digicam.
As far as upgrading to a Nikonos V, they haven't been made in a couple of years and Nikon is getting out of the film camera buisness altogether- everything is going digital. If you really want a film camera I would check out Ebay and pick up a used Nikon RS system for a tiny fraction of what they cost new- somebody's crying over that one.
Right now I just need for someone to upgrade to the new Nikon D200 so I can buy one of their old D70's.
Jon