With drainwater heat recovery you want it to feed BOTH the hot water heater AND the cold feed to at least the shower (but the whole-house is still OK, if you don't mind room-temp water coming out the cold taps.) The heat exchanger presents some restriction, to flow, and some are better than others in that regard (PowerPipe is the least restrictive) but the shorter they are the least restrictive and higher the flow. (I have a 4x48" and don't have any problem getting 6gpm+ through the thing, and that's with some half-inch distribution plumbing and a pressure reducing valve at the meter.)

They make some pretty stubby ones that fit in crawlspaces, and creating a wide section to manage a 3" or 4" version doesn't diminish the drain function but enhances the heat exchanger efficiency. The fatter and longer they are, the better the efficiency, and the labor to install a tiny one is about the same as installing a large one, so if you do it, get the fattest & tallest one that fits. Even a 4" x 24" PowerPipe delivers better than 30% return, and a 4" x 30" returns 40% @ 2.5gpm, according to Natural Resource Canada's third party testing. (At lower flows the return is even higher.) Even if it's not subsidized by the state, if you can fit even a 2x24" in there it will extend tank capacity measurably, and have a return on investment for families with long-shower addictions. It has to be on the drain, not the stack, but it can be located anywhere downstream of the shower. The closer it is to the shower it is the better, but not dramatically better, and even 50 feet away is fine, taking only small hit in measured efficiency.

A split-system heat pump water heaters (mini-split style) work where the tank-top versions won't, since it has the capacity & controls for de-frost. By not presenting a load to the heating system it's true efficiency in a heating dominated climate is higher, independent of it's EF rating.

The Vertex gets good reviews and tests very well on efficiency. The ~75KBTU/hr burner makes for excellent recovery times (it's 2x the burner of typical 40 gallon conventional tanks), and is enough burner to run continuous 1.5gpm showers at Portland's summertime incoming water temps (but maybe not all winter) even without a drainwater heat recovery heat exchanger. Marathon's are great for an electric tank heater, but like all electric tanks (including the heat pump variants) recovery times are poor and the first-hour gallons ratings are quite low compared to even the smallest burner gas-fired tanks of similar volume, so you have to go with a big 'un to go electric. Heating with a gas fired condensing water heater will be literally half the cost (or less) of heating hot water with electricity in most areas. (What are your gas & electric rates?), so even if you're paying $500-800 more up front, the payback on the difference for a typical 4- person family water usage would be under 10 years, maybe under 5 years.