Downtown - Twin 150s without a doubt. I have a 2005 Grady White 257 Advance with 150 Yamaha 4 strokes. I looked long and hard for this specific boat. I fish off the south shore of long island new york and run 60 - 80 miles off (epirbs, rafts, dual vhf's, all the safety equipment and buddy boat) but I also looked at single vs twin application prior to purchasing and the twins performed better hands down (again my opinion). The Single vs Twin debate gets pretty heated in terms of pro's and con's; however, this boat (the 257 Advance) is rated for 350 Max HP. A single 250 is under-powered in my opinion based on personal sea tria of five different 257 advances.
Performance wise, here are specs for the Twin 150 setup on Yamaha's website (test weight specs - the dry weight of the 257 advance is 3,500lbs, plus approx 1,000lbs in motors, fuel, 2 person and 2 batteries gets to a tested weight of 5,981lbs):
RPMS MPH GPH MPG
1000 5.1 1.6 3.19
1500 7.4 2.2 3.36
2000 9.0 3.9 2.31
2500 11.7 5.7 2.05
3000 21.5 7.0 3.07
3500 27.7 9.4 2.95
4000 33.2 12.3 2.70
4500 37.7 15.0 2.51
5000 42.0 19.0 2.21
5500 46.2 25.3 1.83
6000 48.2 28.4 1.7
I can tell you that when I sea trialed it (because I run off shore and wanted to get a better sense for fuel economy), I had a fuel tank of fuel and had 5 200lb guys on board along for the sea trial to simulate the typical 3 guys I run with and additional gear (ice, equipment, bait). We cruised at 3,700 RPMS at 26MPH burning between 10.5 and 13 GPH (seas dictated) which translates to a MPG of 2.0 to 2.47MPG - for an off shore vessell, thats very solid. And while this is slightly lower than the test specs above where they did 27.7MPH and got 2.95 MPG, i had approx 600lbs - 1000lbs additional weight on my sea trial.
In terms of the single vs twins, the Twin 150 setup pushes the boat great and maneuverability was excellent (docking with hardly even turning the steering wheel). Interms of maintenance cost, of course its a bit more, but its not double like everyone thinks. First of all, if you have a bit of mechanical know-how, you can do the basic maintenance yourself, which offsets the cost. For example, most 250 - 300 singles have 6 or 8 cylinders. These are inline 4's and so each has 4 spark plugs instead of 6 or 8 on a single - not much difference. Water separators, engine oil, lower unit oil and fuel filters (non-water separator) are items that you can change yourself and save big bucks - doesn't take long at all. I do, and recommend, having a Yamaha certified tech do the recommended 100HR, 500HR services to ensure the seals and all inside (thermostats and water pumps) are good because it's this preventative maintenance that will get you THOUSANDS of hours on these motors.
Also, the twin setup provides that redundancy both with regard to the ability to get yourself home and while you may not plane depending on how you're propped and how much weight you've got on board, it's way more torque and propulsion than a tiny kicker (that most people don't even use until it's too late and then it may not fire up when needed). Two alternators is nice too so that you can choose how you'd like to ultimately have the boat wired.
In short, I think that this boat is amazing and is a perfect vessell for those in shore days rafting up with buddies as well as the long runs off-shore.
Let me know if there are any other questions that I could try and help with - good luck with the purchase.
