I think apolitical does want to fly it in VIP config!
Not sure if it's helpful merely to quote from Wikipedia, but it seems to say that the wing, MTOW and fuel capacity of the -500 and -250 are the same, and one assumes that full tanks would be easily possible out of an intercontinental airfield with a low ZFW (empty-tank weight).
Wiki also says the -250 introduced an upgraded version of the RB211-524B engine, known as the -524B4 (or -524B4l), which increases thrust from 50,000lbf to 53,000lbf, as well as offering better sfc. (This upgrade is also, it says, available for the -500 a/c, but not how many had it.) So the plot thickens on thrust?
I guess for a given payload, a full-tanks -250 would be significantly heavier than a full-tanks -500. So that would suggest higher fuel flow, later step-climb, and consequent shorter range. However, it would be interesting to know the comparitive fuel-flows at the same AUW and altitude. Would the longer fuselage of the -250 be an advantage or disadvantage from a drag point of view? Are the horizontal and vertical stabilisers different sizes?