With the 60 degree shoulder and shorter neck, JD's version should offer a slight velocity increase over the AI version, but the dies will likely cost a bit more. Even my friend's standard .338-06 runs a 200 Spire over 2500 fps., in his 15" Encore. Sorry, but I don't have personal experience to relay to you on the 2 cartridges in question. My own .338 JDJ No. 2 Contender runs a 200 @ 2280 & that's been enough to satisfy me until I step up to the .375 JDJ.
Tell us about the .338 Cooper Express.