I haven't read all three pages of responses, so I apologize if this has been addressed. But as having owned both a gas Colorado and Silverado, and having a close friend owning both in diesel, I felt I had some insight to share.
The Colorado is a fantastic truck. For its size, it's roomy, and does the job well. I had a 2016 Z71 4x4 with the V6. I averaged 24mpg with mixed city and highway driving. Going off-road, it was very sure of itself and I was able to take it through some nasty stuff - including a pretty bad winter storm in Michigan. I had almost 40k miles on it when traded - nothing but oil changes and the few recalls that popped up. It never once gave me an issue. My dad had the WT with the V6 and put almost 80k on it before it got totaled. He never had an issue with it, just standard maintenance and recalls. I don't recall what MPG he got, but it was a bit better than mine, obviously so. He did tow with it quite a bit, never an issue.
I traded it in for my Silverado, RST 5.3l 4x4 with Z71, because I met my now wife and went from just me and my lab, to me, my wife and three dogs - 1 lab, 1 GSP, and 1 midsized loveable mutt. The Colorado worked, but it was feeling cramped for road trips. The Silverado has just shy of 16k miles on it. Same thing, only oil changes and recalls - no issues. I'm averaging about 21mpg. The Silverado is comfortable, but the leg room does leave a bit to be desired due to the dead pedal.
My buddy bought a GMC Canyon with the diesel soon after I bought my Colorado. The trucks were identical, except his was GMC with the diesel. He paid about 6-7k more for his. He did average about 30mpg. Never had an issue. He traded it in for his Sierra AT4 with the diesel. Last I heard, he's getting upper 20s for MPG. I don't remember off the top of my head how much he paid, and its not true apples to apples comparing a fully loaded AT4 to a mid level RST - but I think it was about 15k difference or so. He's only had it a month, but has taken it on short trips. No issues. He felt the gas engines performed better on test drives, but went with the diesel for the MPG and longevity.
If it were me and my money, I would not get the 4 cyl gas engines. IMO, I just don't see them lasting long term like the V8's will. If you're looking for best bang for your buck, I would get a Colorado with the V6 so long as you don't need the space of the Silverado. The Colorado, depending on trim, may be as much as 10k less than that of a similar equipped Silverado. IIRC, I paid about 35k for my Colorado and 43k for my Silverado. If you do need the extra space of the Silverado, the 5.3l V8 is a tried and true engine that should last a long time - all the while providing fairly decent MPG.
As for diesel vs. gas... IMO it's an investment. You'll pay a premium for the diesel. If you trade your truck in every few years, never breaking 100k miles on them, it's most likely the up front investment of the diesel will not be recouped at the time of trade in. However, if you do plan to keep this truck for a long time, and will be putting more than 100k miles on it, the upfront investment may be recouped. But even then, is 5-8 mpg really that big a difference at the end of the day? To some, yes, and if so for you, then the diesel is the obvious choice. But, I'd steer clear of the 4cyl in both the Colorado and Silverado.