There are a few thousand to choose from. Some of the better brands are Bully Dog, Edge, and Quadzilla. They'll all provide similar power gains, but something you should strongly consider first:
All of those tuners alter how much fuel is injected and when its injected. This will almost always dramatically affect EGT (exhaust gas temps). Putting a tuner on a stock truck and buzzing around town is fine, but towing is a whole different story.
Make sure you do an aftermarket exhaust and intake first. They will dramatically reduce EGT. Install a pyrometer in the exhaust manifold, and use a tuner that automatically de-fuels when you hit a certain EGT (usually around 1100-1200F). That way you won't damage the turbo when you're towing.
Many of them have a selectable tune, some with a knob right on the dash. In conjunction with an automatic de-fueling to keep EGTs in check, they are a wonderful tool. That way if you have it set on #6 for max power and forget to turn it down with a trailer, it will automatically drop it down to prevent damage.
This is the main reason to get an aftermarket exhaust. If you put it on a stock exhaust, the EGTs will climb and the computer will start to de-fuel, limiting the gains you put the tuner on for in the first place. If you use one without EGT-based defueling, then the EGTs will just spiral out of control and damage the turbo.
Aftermarket exhaust and intake have their own benefits as well. Often times you can pick up 30-40 hp just from that.
Every one of those mods should increase MPG. All of the tuners inject more fuel, but they alter when its injected making the engine much more efficient. You end up using less right foot to achieve the same acceleration and cruise. 2-3 mpg is not out of the question if you keep your foot out of it
Not sure about the 24v Cummins, but a quadzilla tuner, AFE intake, and MBRP exhaust on my dad's 04 duramax took him from stock 325hp/550tq up to 600hp/900tq, and EGTs never go over 1150, even towing 10,000 lbs uphill.