I ride a 2014 Kona Unit (rigid ss 29r), currently with the stock 2.25 Ardents on front and back. I ride some fairly gnarly stuff here in SoCal, and also lots of loose, dry stuff over hardback. Thinking about bigger tires for a little more cushion and grip. I know lotsa folks like a fatter tire on the front than on the back--for example, I could put an Ardent 2.4 on the front and leave the 2.25 on the back. But why not put 2.4s on both? Is it just a rolling resistance and/or weight issue? The way I ride a few extra ounces isn't gonna matter. How dramatic a difference in rolling resistance would there be?
(BTW, I currently run tubes. I'm pretty light--I weigh about 134lbs and run around 16.5psi front, 17.5 psi back. Knock on wood, I've never flatted on this bike in 6mos of riding 3-4 times a week with no shortage of rock gardens, so I haven't bothered to go tubeless.)