Rinnai is the world's largest manufacturer of gas-fired appliances. If you were happy with your other Rinnai, you'd be happy with another one. You'd probably be more than happy with a Noritz too. Whatever the brand, local support (particularly installer training) is an important factor. The most common problems with them is usually due to installer error.
In Europe & Asia tankless solutions are popular primarily for the space savings due to smaller home sizes, not higher efficiency.
If you're interested in showering-efficiency a drainwater heat exchanger and a standard 40-50 gallon gas burner would give you pretty the same "endless shower" experience with less maintenance, provided you have at least 5' of vertical drain downstream of the shower.
This is not a solution for one story slab on grade houses. With a 2 story house with the shower upstairs, or a one story with a sufficiently deep crawlspace or basement foundation it works great.
For the average home in TN it's probably not cost effective, but if you're
really running the shower 40 minutes + per day it will have a pretty decent return on investment- much better than the difference between a tank and a tankless. They're effectively zero maintenance, and have a 35-40 year + lifecycle before performance drops to 75% of the day-1 return efficiency. The biggest and fattest one that fits is the "right" one, since the additional cost of goods is outweighed by the higher return efficiency.
It takes a 3" x 60" or longer, or a 4" x 48" or longer to reach a 50%+ energy return efficiency required to make a standard tank into an endless-shower experience. Don't expect to find them on the shelf at the local plumbing supply or box store, but
they can be
ordered direct from the
manufacturers (most of which are Canadian, with at least
one exception) , or through
Home Depot.
A basic gas-fired hot water tank runs at about 80% thermal efficiency, but if you're recovering 50%+ of the heat that would otherwise be going down the drain in 20 minute showers it's better than 120% as-used net efficiency, something a tankless on it's own would not be able to achieve. Of course, you could install one of those with a condensing tankless too, where you'd be pushing 150% net efficiency.