Water Heaters - Water heater reccomendations

We need a new water heater! We are looking for a natural gas model that will provide enough hot water at 8300 ft to fill a full-sized clawfoot tub. Two full time residents of the home with the occasional onslaught of 4-6 house guests. Other than the occasional very hot bath and the inlaws coming to visit, water usage is below average since we are on a well and are careful about conservation. The unit we have now is 40 gal but I wonder if that size will fill my new clawfoot...

The space is the primary gotcha - it's tiny. The unit we buy must be narrow, although height isn't as big a problem.

On-demand is really not an option at this altitude. Apparently the manufacturers that make units rated for our elevation have horrendous customer service/warranties and the units really don't work well anyway (you have to use alot of water to kick them on). The master bath will sit 5 ft from the main water heater so I wonder if a room-specific on-demand heater is worthwhile.

Any recommendations on brands/models/sizes? The plumber is coming to look at our rough-in situation tomorrow and I hope to start the plumbing phase of the remodel in about 2 weeks... (big emphasis on HOPE!!)

Thanks fellow DIYers!

First, the available water from a WH is 70% of gallon size ( due to cold water mix as you draw water out) so you 40 gallon unit will give you 28 gallons of water. If you start with a 140ºsetting, the temp. of the output water will be just over 100 at the 28 gallon point. This is not going to cut the mustard. Add to that that the heater will have to be derated by a tech. from the manufacturer or your gas company, so your performance will be even less. ( (If you are in Colorado, most of the gas is derated by the utility. If you are in another state, you will have to get someone to "fix" the WH to work at that altitude)

In your circumstance, I would say the tub is a luxury. I would consider a hot tub or whirlpool with on-board heater, or just settle for showers!