First thing's first - there is a thread somewhere in this part of SB - think it's under the general board - where someone compares extensively their experience with a high-level PnS and an SLR. At the time, I was using a G7 and two DS125's, and I scoffed at upgrading to SLR. I was very resistant to the idea of going SLR, mainly because wanted to use the LCD screen to frame my shots, it just seemed like not much of a difference except for the $.
Then one of my dive buddies bought an SLR, and almost immediately, her pics seemed better. While high-level PnS's do a great job, the thing is, with an SLR, you are using dedicated lenses and can control the overall sharpness a bit better, it seems. I am sure I'm gonna get crucifed for saying this, but...that's the way it looks to me....
One Friday night, after a couple of beers and some prodding from my husband, I took out the plastic and bought a whole used SLR setup, BAM, hyperventilating on the couch for about an hour afterwards. And after one dive, I was a CONVERT. I actually LIKE the viewfinder, it seems like you are more "in" the shot you're taking! When you press the shutter, it closes immediately, there is absolutely no lag time!! And the crispness....ohh. I went from the G7 to a 20D - fewer megapixels, but my shots are crisp when I crop them, and I can really get a lot more shots in the same amount of time.
All this said, keep in mind what you're suggesting, moneywise. You think it isn't that much more expensive, but it kinda is. It isn't just camera plus housing plus strobe. Add onto that various ports you'll need for the various lenses you'll also need...
Whatever your decision, good luck with it, and have fun.