Upgrades are much harder to come by on CO than NW. Over the last three years, I have never missed an upgrade on an eligible flight on NW, but am batting around 50-60% on CO flights. Furthermore, while NW almost always upgrades me at or before the five-day window, CO frequently makes me sweat it out until much closer to the date of travel. With that said, the food and seats in first class are substantially better on CO, and CO, unlike NW, actually shows a movie on many of its longer flights. On the other hand, if you get stuck in coach on a 733/735/73G, you will find that there are far fewer attractive seats than you might be used to on NW. I especially dislike the 73G, which plies the DCA-IAH route frequently, when riding in the back because the entire bulkhead row 5 is pretty worthless, making only the landlocked window seats 14A and 14F, with virtually unlimited legroom, worth a dang. One thing you should definitely watch out for is your fare class: Q, I, S, W, T, X, and L fares will only earn you 50% elite qualifying miles in the WorldPerks program, which is definitely a bummer, and likely to be a problem if you are trying to book the cheapest possible fare.
As for your routing, you should be forewarned that DCA-IAH and vice versa is brutal for upgrades. I have sat in the back as a Platinum on both Y and H fares and my understanding is that my experience may be the rule, not the exception. IAH-LAS offers you an improved chance for an upgrade with bigger equipment and fewer business travelers, but even it is far from a guarantee. I have no idea about EWR-LAS. If you want to play around and see how elites have been doing lately with regards to upgrades, you might try plugging in some sample flights into the new
CO PDA tool and check the upgrade standby lists.