I shoot and reload the 6.5 JDJ in both 14" and 21" barrels. With light bullets it's great for larger varmints. With 120 or 125 gr. bullets it is excellent for deer. I get about 2400 fps with the 125 Nosler Partition using RL-15, with excellent accuracy, from the 14" barrel. Seems to me it's around 2600 for the 21" with this load, but I don't have my notes handy. Reloading data is available in some manuals, and SSK Industries will send some if you ask. In my opinion, the .257 might be better for varmints due to the availability of lighter bullets, but the 6.5 seems better for deer, though either would work I'm sure.
Hope this helps.