I wold go 6HP too as its the same weight as the 5HP with a bit more power.
As for Mariner vs Yamaha... I guess its your choice..and maydepend on your local dealer for servicing or parts..price etc.
I was in your position three years ago and bought the Yamaha 6Hp and am still delighted with it.
The selling point for me was the Yamaha can be carried in my car on all three sides .. and the rounded handle is on the fourth side (end) so I cant accidentally carry it the wrong way and risk engine oil flooding into the head. A lot of small 4 strokes can only be laid on one side.
I have found the Yamaha very reliable..easy start ..easy to service myself. So not complaints with it. So saying I have Mariner 2 strokes and they are reliable too