I've been in the boat sales business for 14 years. Noone builds a better outboard than Yamaha nor a better resale product. I have personally had good luck with Mercs, but, Yamaha is a better motor. This is particularly true of the bigger HP motors.
Worked in a commercial fishery for 11 years, and I tried them all. Ran mostly 9.9 hp and 250 hp motors with some other sizes mixed in. Yamaha without question outperformed and lasted longer than all others. At one point I had 54- 250 HP Yamaha outboards for our fleet of boats. Time was money and should a powerhead blow on a motor (even on Yamaha's this happened) the bad motor was off and a new motor was on in under 1.5 hours.
Yamaha C series or "enduro" motors are also bulletproof and a good choice.
Wow I'm suprised there aren't more Honda guys (or gals) out there! I'm an Evirude fan since Bombardier took over the operation but I've been on many a boat with Honda 4 strokes and the Honda is really an incredible engine.
I used to have an older 10 hp Honda 4 stroke , but needed a bigger motor, But I have to say that was a great motor for trolling . Loved it. Now I have a 25 yamaha , still a great motor but I miss the 4 stroke .
I purchased a 135 Opitimax new in 1998 and have had no problems. Moved here from SC (sea level) in 2000, just turn the key and it starts and runs perfectly at any altitude. I have just had to reprop. Regardless of make I would suggest you aquire a motor with some sort of EFI if you plan on boating at different elevations.
I got a 5hp from briggs&stratton 4 stroke,for my 12'jon boat and it dose a very good jop for a 5hp.Good for trolling too,took it up to Horsetooth last septemper went from the boat ramp on the west near the Camp grounds all the way up to the dam on the north side and back within six hours,going slow, used very,very little gas,and I got it almost a hundred bucks cheaper than Cabelas or Sportsman Warehouse,with S&H $612.00 two or three year warranty on it.Brandnew.Get's you in and out of the water if you need too,it jetplanes my small boat all the way when you run it on full power.LOVE IT.Ohyha and it is automaticly air cools,dosen't get hot at all.
On the TR-21, I have the Yamaha VMAX HPDI 300HP, on the Commanche, I have the Evinrude E-TECH, 250HP. They both have excellent holeshot and awesome top end around 68 to 73 MPH depending on what altitude you are running in.