Easy. I've driven cross Country more times than I remember and the simple rules I've always lived by is:
1) When you have to stop for a break (bathroom, food, stretch, etc...) fill up with fuel.
2) If you see a sign that says "last gas for 200 miles"... it probably isn't lying, stop and top off.
3) Pick a time you will stop driving for the day. Pick it earlier than you think (I usually start looking around Supper time.) That way if there isn't any room open, you still are fresh enough to go a bit further. Don't start thinking of finding a place when you are already tired, that is too late. If you find a place early, go to bed early and get an early start the next morning (cooler for longer and less traffic).
4) When you do find a room, top off the gas tank that evening. If you get an early start some gas stations in smaller towns may not be open when you are ready to hit the road. While you are getting gas, buy some snacks so you don't have to run down to the vending machine and pay 4 times as much at the Motel. (get enough for the next day on the road as well.)
Cell phone reception. Yes you will lose it in the more rural areas. Don't get your knickers in a knot, people traveled long distances long before cell phones. Think back to how many times your vehicle has left you standing by the side of the road calling for a tow. If that number is below 2, you probably have ZERO to worry about. If you are in an area that has spotty reception, the locals that travel that stretch of road know where they are and are generally more apt to stop and offer assistance if you do find yourself stranded.
Either route you are looking at is a great drive. While I really like Colorado myself, it is hard to beat some of the lonely regions of the other route you mentioned. It is open, bare, and starkly beautiful. I couldn't live there (I like trees too much
), but I love driving through those areas. Some of the areas in Montana are simply beautiful, so much so that I seriously thought of moving there 5 or 6 years ago.