Boilers are designed for radiant heat and have radiant controls on them. They are more efficient and run at about 84%, at best, for non condensing units. But they are also a lot more expensive than water heaters.
Water heaters, for a specific and relatively small area, like a garage, can work just fine. They run at about 75% efficiency, but they remain hot even when not heating, so the best thing is to turn them off in the warm season. Don't combine your domestic hot water with your radiant water. The radiant should be a closed loop.
The brand doesn't matter much because corrosion and sediment will not be concerns and most tank water heaters are basically the same design.
For a normal heating system you can figure about 25 BTU/ sq ft. So a 400 sq ft well insulated garage, for instance, only needs about 400 X 25 or 10,000 BTU. Double this would mean better recovery. So if you wanted to use a standard water heater with a 40,000 BTU burner and 70% efficiency, you'd have 40,000 X .70 = 28,000 BTU available. Plenty! Adjust these numbers for your size shop.
There's more to the recovery rate story than simply BTUs at the burner. So take "slow recovery" warnings with a grain of salt and do your own research.
There are some very useful tips to simplify the systems in cases like these that can really reduce the overall cost and up the reliability. Simple is good in some areas that are smaller and not always used, or where cost is a major concern.
If you want the best efficiency though, get a condensing modulating boiler in the 100,000 BTU range and set up a full hydronic system.
This assumes you'll be using natural gas or propane.
