Being used to much bigger more powerful bikes, My perspective on the Rebel's highway performance might be a little biased against.
However, I'd say that it is remarkably capable for a 250. I find that holding on to the handlebars for support against the wind on the highway gets quite tiring after half an hour or so. Also at highway speed, the throttle is basically all used up, and its necessary to just hold it essentially wide open. I say essentially, because there's not much reaction to the last little bit of throttle, and I seldom have the throttle literally WFO, just nearly so on the highway.
To respond to the question asked, the 250 is a 40 year old design. It was already semi-obsolete even in 1985 when the first ones were built. The 300 is a brand new design with a modern engine. I have never ridden one, and I don't care for the fat tires and other design aspects, but I am pretty sure that the 300 would be much more capable on the highway, and if I had to pick one or the other I would opt for the 300
2004 Rebel 250, 2003 BMW K1200GT (roadburner), 2004 BMW R1200GS(all purpose),
1973 Norton Interstate (in a box in the basement)
1968 Triumph Bonneville