What ballvalve said. With the huge standby loss of LP-fired tanks the as-used efficiency factor in this low-volume use situation will be quite low- less than 50% efficiency even with a power-vented version. The cost of a condensing tankless up to the job of supporting a mid-winter shower would be astronomical compared to the actual savings standby loss.
At current LP & electricity pricing in CO, an electric tank is the way to go. It's standby loss is already quite small compared to an LP burner, and can be made much lower with additional insulation.
Don't stop at just insulating the tank itself- roughly half of the standby on an electric tank is heat conducted out via the near-tank plumbing, so 5/8" wall closed cell pipe insulation on everything within 10' of the tank (including the cold feed and the t & p valve outflow) is called for. In new construction you might as well put in on ALL of the hot water distribution plumbing. (It's now required by code in some areas.) True Value sometimes carries 5/8" or 3/4" wall thickness pipe insulation for half-inch pipe, but otherwise it's available online, Graingers, or plumbing supply distributors. Home center box stores only carry 3/8" wall goods.