The 2500 in size and layout is the natural evolution of the 2000XL. As far as upgrading is concerned JJOS2XL and JJOSXL are available for the 1000 and 2500 respectively. (Despite the naming differences they are essentially the same thing).
There are also larger sized replacement LCD's available for both units at MPCStuff.com. These include a free version of JJOS and a fuller featured XL version for those screens is available for purchase.
The thing to note with JJOS2XL/JJOSXL is that the 1000 essentially can get all the features of the stock Akai 2500 OS plus adds a ton of new functions. Basically a cheaper 1000 compared to a 2500, both with JJOSXL added, only differs in lack of tilt screen, different shaped pads, and 2 less Q-link controls, and the lack of numeric buttons.
The pads on the 1000's even the newer replacement ones (actually a return to the design used in the 60/3000/2000/XL) have a bit of a gap between the pads and the sensor sheet, so you need to do a DIY fix like applying silicone to underside of the pads or buying MPCStuff's Fat Pads for around $40. I have them, they're very nice.
You'd probably like the Ren over Maschine as Maschine's sequencer and workflow being different than an MPC seems to be the main thing that keeps some users from gelling fully with it. Obviously the Ren's entire philosophy is to recreate the MPC sequencer and workflow in a software environment, so I suppose the question becomes do you want to continue having your MPC be totally standalone or move to the hybrid style and gain all the computer based amenities but lose the standalone independent operation of the hardware MPC's.
1000/2500 with JJOSXL are very capable machines and already having a hardware MPC it should be easy for you to weigh the feature set and additions they offer over an older MPC like an XL.
Here's the JJOS comparison charts. (Akai OS/JJ Free/JJ OS1/JJ OS2/JJ OS2XL). When you buy XL you get access the install the OS1 and OS2 for the purchase price of $123. All the paid OS's can be previewed any time, save is disabled until your purchase an unlock code.
http://www7a.biglobe.ne.jp/~mpc1000/mpc2500/The unlock codes are machine specific so you can't resell the code or transfer it to another machine. This isn't all bad as often you might find a used MPC that already has a paid JJOS in it. (Usually reflected in the selling price, but of course if you ever need to sell a unit with a paid JJ OS, you can also have that command a higher price).
Ren is new so those forums have lots of info/feedback daily, and the archives have all you ever need to know about 1000/2500/JJ OS. And there are still lots of users of those machines posting regularly.
