an extra 100 watts and 50 khz signal= best money you could spend at twice the price, I can't begin to tell you how much structure and how many fish show up on one frequency but not the other, with just the 200khz signal you are literally missing almost half the picture, and in the salt power is everything.
I have said before I have a 200khz/50khz 200 watt alpeco(raymarine)265 that I'll let you have for 75 bones, and I would love to unload it, but if you fish in salt I can garauntee you don't want it. My garmin with 500watts is the best buy I could have made. I think I spent an extra 2 or 3 hundred on my plotter/sounder to get all the features I wanted and it was a bargain.
Buck up and spend the coin on what you know you will be happy with.