Twin F150's are the way to go. You stated you fish offshore, although from Maryland the run is not super far, having the reliability of twins is almost a must. If an engine breaks down you can limp in on one. F150's are very fuel efficient and allow plenty of power and are still rather fuel efficient. An F350 would power the boat with plenty of power, but it gives you only 1 engine, tune ups and service are a lot higher on an F350 then an F150, I bet both F150's will not be a whole lot more to service then the single F350. I believe the F250 is roughly $200 more at my dealer to tune and service, maybe more, parts will cost more for sure. As for trolling offshore, twins are better, if you need to turn on a fish you have more manuverabilty and when you fight a fish you cna just keep one in gear and move slower to keep tension on a fish rather then moving faster with the F350. Manuveuring in the dock will be a huge benefit if you have twins, they spin a lot easier and will get into a slip a lot better. For near coastal fishing a single would be ok, although most guys I know who even fish near shore regret getting a single and wish they went with twins. F200's would be overkill on a 23 gulfstream, the F150's are the best package and the only way I'd by a gulfstream, especially if your running offshore.
also, if you do not plan to always run with a buddy boat, twins are warrented even more. If your running a single you really should have a buddy boat on all offshore trips. Another side note, the F350 has a very high cowling, which makes swinging a tuna even harder. Brackets are bad enough IMHO, but having a big cowling like an F350 behind the boat makes it even harder, the F150's aren't too bad to get around.
As for the difference in efficiency, your talking maybe .2mpg at most change between the 2 engines, at most speeds it is less during cruise so your range will not chnage a lot and your over all bill is not going to be a significant difference, If you were talking .4mpg or .5mpg difference I'd agree there might be reason to go with the more efficient one. There are a lot of rumors that the F350 is not really producing 350 hp, not sure if it is true or not, but for the hp they are heavy engines. If you go with twins you will have no regrets, if you go with the single you may wonder. I went from a single to twin powered boat boat during our last purchase and the difference in handling, manuverabilty and safety are well worth it. Had to limp in once last year on a single after braid got wrapped in the prop, limped with it raised, pulled it in the slip and got it off without any damage. Couldn't do that with a single unless you want to get very wet or kill yourself hanging over doing it upside down.