As I mentioned on the other thread you have going, I would rather buy one with 400 hours on them.
800 hours doesn't seem so bad on four strokes, but if they were HPDI's I think that's quite a lot of hours. I would probably buy them anyway though, and just take care of them.
When you have another option for half the hours, why not go that way? I know, more money. You said about $8K more? Looks like you might spend quite a chunk of that difference on the older motors anyway.
Don't forget about the trailer - they can be a money pit too, especially if used in salt water. Take that into consideration on your comparison of the two boats.