I love both machines. Use both heavily. The one advantage the 2500 has is audio tracks which comes in handy when you use Virtual/software as sound modules.
Other than that if you have JJOSXL they pretty much do the same thing. JJOSXL will get the nod with workflow...The 4K will get the nod with better sound quality and overall better results when it comes to things like Portamento, resampling, tuning, etc.
Can't go wrong with either machine..IMO. With the 2500 you'll need to do some corrective mixing to get a professional sound. With the 4k I really dont have to do too much to the mix once I get it in pro tools. The shit sounds beautiful to me. That's just my experience though.
4K will get you 512 MB Ram for memory consuming MULTI samples...USB for Memory sticks, keyboard to type file names, external HDs etc. also it has ADAT and a wordclock board to stablize the digital connection. 4k's the real deal.
