the biggest difference between the diablo and the bully dog is that the diablo offers custom tuning and the bullydog is a "canned tuner" meaning that its a broad kind of standardized tuner. you can change your shift points and rev limiter and top speed limiter...it will give you a good increase in hp/tq and a good gain in mpg's (this is the tuner I run. with the diablo you can get a custom tune for your truck and the mods you have placed into it.
for example if you get Longtube headers or something of that nature you can tell the tuner to download a special tune to squeeze every ounce of power out of that mod. if I could go back I would have gotten the diablo and I still plan on doing that.
if you plan on doing a lot of modifications to your truck I would highly suggest you purchase a diablo (custom style tuner) but if you are only going to add a tuner alone the bully dog will work just fine for that application. ive had my BD on for about 50,000 miles and ive had no problems with it. hope this helps