So just from a power stand point, yes there is a big difference. You said you have trouble getting to 65mph. On my 500 with my wife as a passenger we weigh basically 300 combined. I can get to 85 without an issue, and probably faster really.
As far as ride height. I don't think changing the preload is going to do much if anything for the seat height. So if it ends up being to high you may have to look into other ways to lower the bike.
I have ridden a Rebel a 250 a bunch of times and it is certainly a light bike. That being said the extra weight of the 500 really shouldn't cause an issue. The bike is so well balanaced it really feels very light.
The 300 is a single cylinder, so if you didn't like the 250 buzz you may not like the 300 since the 250 is a paralel twin. The 500 should be a smoother.
That being said, while I myself would without a doubt recommend the 500, the 300 engine is actually fine as well.
It has 50%more horse power than the rebel 250. And while it takes a bit more to get it up to speed than the 500, the 65mph speed you are looking to attain actually wouldn't be a problem.