[Bill]

To give you an idea, I have a direct comparison from my peak in '03 and my peak in 04 after I broke the 40. The time for a 50 meter dive averages about 6 seconds more with the 30s, but I dive more relaxed and added 5 meters (and 15 seconds) to my PB. I vote for the 30s too.
You'll probably want to use a mono before you want the 40s. Worst case, if you take care of them, you'll get most of your money back on e-Bay. There still the best spear fishing fin around.