I've ridden 250's and my 310 back to back and there's no comparison. Each has there advantages/disadvantages. You know you're riding something different when you're on the 310 and it's probably very similiar to the threads between the KTM 250 and 300. Do a search on those for more related info.
One thing you notice right away is that the 250 feels more like a 125 (after you've hopped off the 310), because it has a distinct powerband and more top end rush than the 310. Plus, at full tilt, it's more manageable than the 310. I would venture to guess that a 265 would be more of everything that the 250 has.
On the 310, the powerband just flat out changes. There's more low end, a lot more midrange, and less top end then the 250. I have the stock gearing on my KX and it pulls each gear very hard. Once I richened up the jetting and retarded the ignition timng, it smoothed out the midrange and gave me more top end, but not as much as a 250. I still have the stock pipe on there, so if I want more top end it'll probably require a pipe change.
Once you adapt you're riding style, however, the 310 is a joy. I find myself going into corners a gear or two higher than a 250 and just feeding the throttle with a slight application of the clutch and carrying good momentum. Although you could be Don Garlits from corner to corner, this motor (much like a mini-open bike) rewards momentum and a workmanlike(ie more sedate) approach.
You've probably seen my other posts on a flywheel weight. I'm thinking of doing this to only enhance the tractability even more.
Tough choice. If you want more of everything your 250 has, I think the 265 with porting is the choice. If you want something totally different, the 310 (which also has porting done to it) is your choice.