Definately, neck up from 243. You can neck down from 308, but necking up always seems to be a bit less troublesome. I'd recommend either a tapered expander ball (Redding makes them, as I suspect most others do as well) or a tapered mandrel like the ones that Sinclair offers. Personally, I like the mandrels, but that will involve a separate expander die ($24.95, their catalog) if you go that route. A quick brush lube in the case necks with a little Imperial, and you're in business.
If you do go the 308 necked down route, just watch your case neck thickness and ensure that you have proper clearance when the round is loaded. This can sometimes require inside reaming or outside neck turning if there's an interference question.
For .260 brass, I use necked-up Lapua .243 brass, made by runing the cases through a tapered expander on my Redding neck sizer die. I use The Lapua brass for my H. P. Silhouette ram loads and have had no need to turn the necks or trim the brass. Some people caution about forming a "donut" at the base of the neck when necking up lapua brass, but I've never encountered no steeking donuts.
I'm developing a .260 long range hunting load an I'm planning to do a minimal skim cut on the outside of the necks to assure uniformity, but doubt it will be of much help with Lapua brass. Certainly worth doing with other brands, especially Remington. Winchester brass is a little more uniform than Remington, and a little less than Lapua.