I would avoid investing in a platform outside Masters or Epiq. Add ons are smaller (but cheaper) with less variety. Classic series stuff usually has good trade in as they are easy to re-sell, as they are smaller with a lower initial sticker price. They can also be resold on the private market easier.
KRL (10 prefix) or Epiq would give you another set of 4 inch drawers, and 6 inches of depth on all drawers. What's your specific issue? Organization and layout? Bulk space? Larger/deeper drawers? Specialty tools? Do you buy your own scanners/scopes/specialty tools?
It's all about money though. I talked with SO Saturday and got a locker for my KRL1033, and decided not to trade my other KRL stack in either. It was that or trade in everything I have on 2 Epiq 84 inch boxes. The offer for the 2 epiqs I thought was fair, but looking at a cubic inch perspective, and overall organization, the bulk storage of a locker was superior. I would have liked the double 84s (bigger than a Mr. Big

), but the dollars/space didn't work for me.
As a point of reference - A classic locker is 3k, a KRL is 3800. KRL has an extra sliding drawer, 6 inches and width and depth, a total of 11,000 cubic inches more. $800 gets you from 17k in^3 to 28 in^3.