The Girard tankless water heaters are roughly half the price of the RV 500 but there are a lot of tradeoffs, some good, some not so good. The Girards will fit in the hole left by a 6 gl tank model and has doors available to cover the hole left by a 10 gl unit. The RV 500 needs the hole and space left by a 10 gl tank. If one is removing a 6 gl unit and there isn't room to open up the wall to a 10 gl size, then one is limited to the Girard.
The Girard appears to be a good unit. I saw it demonstrated a month or so ago and it put out enough hot water for a shower. It is about half the price of the RV 500 but achieving the smaller size and lower price comes with some compromises. To keep costs and size down, the Girard has a two stage burner without a variable gas valve (such as the one in the RV 500) that regulates water temperature when flow and and incoming water temperature vary. To control water temperature with the Girard, one first must not try to mix the hot water and cold water. Instead, turn on the hot water only and adjust the water temperature by adjusting the water flow. The slower the flow, the hotter the water and the faster the flow, the slower the flow. The Girard rep that gave the demonstration said that modifications to the RV's water system may be needed to ensure enough flow to regulate water temperature. Several RV manufacturers have been installing Girards in their units.
The RV 500 is more expensive but operation is fully automatic and it puts out water at a consistant temperature that can be used in the same way water from a conventional tank water heater is used. The RV 500 would be more suitable in situations where more than one person and/or appliance may be used at one time. If running on tank water, it will also allow more control of the amount of water consumption. It should also be more suitable for use with a washing machine. Even the Girard rep admitted the RV 500 is a better unit than the Girard, saying that the Girard's advantage is lower price and smaller size.
The downside of a RV tankless water heater is gas consumption. Since it doesn't have to maintain temperature in a tank of water, it will use less propane than a tank type. The reason propane consumption may increase is there is no provision to heat water with 110v as a tank type can. If one is used to heating their water with AC, then gas consumption will increase.