Imo, since your limited to just the 2 Yamaha choices, it is a tough call....
2 x F200's = 1166 Lbs., (583 ea).
2 x F150's = 932 lbs., (466 ea).
Difference = 234 lb.
Ideally, 300 hp, (twin 150's) should be ok for that GS. However, two things about the Yamaha F150 that would concern me.
1 - I've read some reports that they weren't actually putting out 150 hp. I forget the numbers, but something around 140, at the prop. I've also heard there was a class action law suit against Yamaha, because of that false hp claim. True/false? Did Yamaha make any changes to the 150 since it's intruduction?
2 - they are 4 cylinder engines. That would scare me a little, because all of the prior 2 stroke engines are 6 cylinder, when they make the jump to 150 hp. So how much torque and true power are you losing right there?
The extra hp, (twin 200's) wouldn't scare me at all, in fact I would prefer it. it's the extra 234 lbs I don't like. How does the boat sit at rest with the 200's, (lower in the stern)?... especially when you have a lot of fuel in the rear main tank and little in the front auxillary tank, (if that's how it's set up). How's it's ride... do you need more trim to get on plane, (vs 150's)?
I know GW lists performance info with different set ups, but if it were me, I'd want to see it in the real world. I'd want to see each boat in the water, at rest, as well as a serious test ride on each boat with the 2 engine options. Try to have 3 or 4 people on board, and a decent amount of fuel, (weight).
The bottom line, (imo) if the boat sits ok at rest, planes out and performs ok with the 200's, that's what I'd go for.