Can’t argue with your assumption that the 283 may be a little easier to push. However I have a 30 Marlin (same hull as the 306) which is probably a little heavier than that 306 and the 250’s move her along at 26-28kts around 44-4500 rpms.
… one thing is almost for sure, you won’t regret getting the bigger boat
I'm not positive but it looks like the 305 Express and the 300 Marlin have the same hull but the cockpit and some other design features are slightly different? Anyway, here's a link to Yamaha's website where they have the numbers for a 2009 Grady White 305 Express with twin F250's on it and it does a top speed of 45.6 mph with the optimum cruise being 28.9 mph @ 4,000 rpm while burning 22.5 gph of fuel. Doesn't sound like terrible numbers or anything until you read the small print below which says "Test weight includes 1/2 tank of fuel, 3 batteries, safety and test equipment." Which to me, the 305 Express, along with the 306 Canyon, both have a 290 gallon fuel tank, so an extra 145 gallons of fuel at 6 lbs / gallon would mean an extra 870 lbs just for fuel to have a full tank. On top of that, if you had any extra gear, food, ice, bait, tackle, persons and so forth and had a full tank of fuel you're probably going to be at best in the mid 30's mph range and your numbers will likely be nowhere near be as great. To me, and this is just my opinion, it seems like that setup would be a bit underrated in terms of power, depending on what kind of fishing/boating needs you have.
To the OP, what year are the motors? If it's anything up until about 2009 or so then make sure you have the exhausts and mid sections checked for the infamous Yamaha corrosion issues before you buy them. If you don't, it can be upwards of $2k or more per motor to have them fixed, of which that's if the motors can be fixed and if the parts aren't backordered. Good luck with whatever you decided on doing. I couldn't tell you about the extra space of each boat, though I'd recommend looking at each in person to see which design or layout you like better.
yamahaoutboards.com