I replaced a 25hp Honda that was nothing but trouble with a new '06 Yamaha 25hp 2 stroke with electric & manual start. It is without a doubt the best outboard motor I have ever owned. 100-1 oil mix & very fuel efficient. The motor weighs 111 lbs & has been extremely reliable.
I would have chosen the two stroke and taken the fuel consumption hit for reliability and self service and self diagnose too. I'd run 50:1 oil. I'm a current auto electrical mechanic. Another 10 years and the 4 strokes might be there for me & my transportable boat scene.
Reliability lies in the ability to make your own repairs on systems when they break. And they will break. Usually when and where there are no repair services available or they're so expensive it doesn't make sense. So that's the main reason I choose the 2 stroke, its easy to diagnosis and repair. I too run 50:1 ratio even though the Yamaha 25HP calls for 100:1. I think it's just safer and adds to the longevity of the motor.
I think doubling the amount of oil is a mistake. The plugs will foul quicker, there will be more carbon build up which will actually reduce the life of the motor and there will be less complete combustion which means you'll end up dumping more unburnt oil & gas into the water. Yamaha would not spec 100-1 if it wasn't what was best for the motor. I have a lot of hours on my '06 motor at 100-1 & I've had no problems.