Its rough to justify diesel these days unless your going to work it hard. I only owned gas trucks and bought my older Cummins about 2 years ago. I love the truck, my wife hates it. She doesn't like the sound or the smell.
I personally wouldn't do the deletes on a newer truck unless it was out of warranty and even then, I probably wouldn't unless things started going bad, such as EGR and DPF.
My truck is an 04 with a 5.9 and no emission equipment. It has 4.10 gears and it feels pretty quick to me. I have an Edge chip on mine and it wakes it up a lot. With the chip set to level 2 (level 4 is the hottest) it is much quicker off the line with less boost lag. Of course turning a truck up to much starts breaking stuff. As far as the gears go, the six speeds have much lower gears and more of them so 3.43 rear gears isn't that bad unless you tow heavy.
As far as fuel mileage? You here a lot of stories of this great fuel mileage from a Cummins. I think most of them are stories. A first gen truck would probably haul down a honest 25 mpg but it was also rated at something like 165 horsepower. In my experience, I average around 15 mpg, maybe a little better. I don't do a lot of pure highway driving but I don't think I would ever get much better than 18 mpg on the highway. My experience with my past gas trucks is I get slightly better mileage on the highway with my Cummins and about 3 mpg around town. Towing a 10,000 load, you can expect to get at least 3mpg better with a Cummins and maybe as much as 5 mpg better.
Maintance cost is a toss up in my opinion. Other than change the fluids, I haven't had to touch the engine or transmission in my truck yet. I redid the brakes, and had some front end work done. I also had some rust repair done. I'd say I have put close to 5k in my truck since I got it. A lot of the work I did my self but most of the cost (3k) was in the body work. Non of the work I had done HAD to be done, but it needed to be done in my opinion. Most of the stuff had nothing to do with the truck being a diesel but more the fact it's a 3/4 ton. Stuff just costs more, such as the brakes. Resale value is also very high for Cummins trucks in my area.
To be honest most of the vehicles I've bought over the years I got because I want them, not because I need them. I use mine as a truck, but a 1/2 ton would work for me, I've just always wanted a Cummins and am happy I got one when I did.
2004 3/4 ton Quad Cab, 24 valve 5.9 Cummins