I made this decision a few months back and chose Snap-on KRL. I use Lista boxes at work, we have models from the 1980s until 2011 and every year in between. The newest boxes are really nice, our old ones are all broken down junk.
Lista benefits:
-The drawers roll very smooth, they're much smoother than Snap-on.
-The load ratings, per drawer, are higher than Snap-on KRL
-I would say the boxes themselves are sturdier than Snap-on. The steel might not be thicker, but the design is better so the drawers are stiffer.
-Their drawer organizers are very nice. Every drawer has slots for the organizers already built in, and they're very flexible.
-The list price is cheaper.
Why I chose Snap-on:
-The KRL drawers come with liners and they're awesome, Lista boxes are bare metal.
-The Lista drawer rollers are plastic wheels. The old ones where I work are all blown out, and they're riveted in place so they can't be changed easily. You can possibly replace the whole slide though. Snap-on KRL slides are warrantied for life, or you can buy new ones for about $60 per pair.
-The bottoms of the KRL drawers are smooth. If your drawers are full and a tool gets kicked up, it won't get stuck on the bottom of the drawer above it. The older Listas had a bar under the drawers, the newer ones have holes, ribs, and the screws from the organizers.
-The Snap-on KRL will allow you to open more than one drawer at a time. The Lista boxes are designed for the dumbest guy in the entire factory, so he can't open 2 heavy drawers and topple the box. If I want to open my drill bits drawer and then open my calipers drawer to measure a bit, I can. Lista owners can't.
-Snap-on KRL boxes go on sale. I got my KRL722/791 combo brand new on the truck for around 45% off of list price. It took some luck and waiting around for sales, but even the average guy can probably negotiate 10-20% off on a $6k purchase. Who are you going to ask for a discount on the Lista? The industrial sales rep who just sold $300k worth of boxes to the local GM plant?
-Resale value. When I was shopping, none of my friends had hear of Lista and they're machinists. My mom recognizes Snap-on! A KRL box will sell itself if you hit hard times or want to upgrade in the future, a Lista will not.
-Used accessory availability. If you want to buy a locker or top chest, there are KRL versions on ebay and craigslist every day. Lista boxes are extremely rare on the used market, and there are so many options that finding a matching piece would be impossible.
-The Snap-on KRL lock'n'roll mechanism is the full width of the drawer. The Lista rolling boxes only have a small tab on the right side of the drawer. The Harbor Freight 56" box and 5-drawer cart have copied this design, I hate the HF version and I hate the Lista version. This is especially important on the wide drawers, where it makes sense to pull them from the center but the latch is all the way on the right.
http://www.harborfreight.com/56-inch-11-drawer-industrial-roller-cabinet-67681.html
If I had to make the choice again, I would choose KRL every time

That said, Lista makes a very nice box and you will be very happy either way.