I know that a lot of people have some bad stuff to say about Sears, particularly with respect to whole-house HVAC systems, but I have to say that they did right by me with an electric water heater replacement.
We had one in the apartment over our garage blow out one night, with water pouring through the ceiling onto the cars and other stuff below. Killed the power, cut off the water. Went on the Sears web site the next morning. Saw that they have a same-day installation if you order before noon-ish. But you have to call, not order online. They wanted literally half what my local plumber wanted to provide the heater and install it. He would install a State heater, they would install a Kenmore heater manufactured by State. So similar product, much lower markup on the heater and much lower installation charge.
The guys they sent to do the job were a well-respected local-ish mechanical contracting firm that is licensed to do both electrical work and plumbing. This wasn't some "tech" that came out; it was a licensed plumber and electrician. The mechanical contractor recommended and I agreed that we should install a drain pan under the heater so when it eventually fails again, no cascade of water into the garage. Too bad my trusted local plumber had never thought of that. The drain pan was another hundred or so; well worth it, and they did a meticulous job of penetrating the exterior stucco without damaging it to install the drain. I really couldn't be happier with the guys that Sears sent, and, like or lump it, Kenmore is the most popular brand of water heater (not saying it's the best, but it is the most widely-installed brand). And like I say, this one was made by State, a standard brand.
So I of course don't know about Denver, but if I had to do it all over again, I would call Sears at 1-800-549-7853 which is their number for same-day water heater installation. It looks like on the web site they have a 6-year, 40-gallon water heater, natural gas, for $398.99, plus $239 for installation in Denver, plus $35 for a permit. $129.99 more if you need a new expansion tank. Don't know what the charge is for emergency installation, and the selection of water heaters available for same-day installation might be more limited, but that's their standard fee, which would set you back about $675 all in. About half of what your Rooter guys wanted. Similar to my experience.
Hope this helps. Whatever you do, let us know how it turned out.
By the way, the 6-year in home repair agreement on the water heater I mentioned is $83. We have repair agreements on a number of our appliances and the service has always been excellent. They show up, diagnose the problem, order the part f they need it, which comes in a day or two, then come back and install it. Not completely optimal -- you'd love to have that range repaired immediately -- but it's a cost-effective approach that has worked well for us over the years.
And, for what it's worth, I just had a look at some other threads about water heaters on here. One of the pros -- Dunbar Plumbing -- actually recommended Kenmore gas water heaters a couple of years ago, saying he prefers them because their design and specs on R-values are better than the competition.