If you go with a gas heater, I would recommend a direct vent or a concentric vent heater since you do your brush cleaning in the garage. These types of units draw their combustion air from the outside.
Rinnai makes a unit that would not break the bank and handle that single bathroom with no issues if you are interested in tankless that is. Its the R50LSi it is a concentric vent unit. One thing is you have to make sure the gas piping to the garage can handle a max load of 150K BTU's here is the link for this unit. http://www.rinnai.us/tankless-water-heaters/r50lsi/
Now if you rather have a tank heater, Bradford White DS1-40S6FBN. This unit is what they call a direct vent that uses the concentric vent piping. It draws the fresh air from the outside and has a sealed combustion chamber. Max BTUs is 42K so gas size piping is not a big issue here. These water heaters cost about double of a standard vent heater, but it is worth it for the safety of getting its combustion air from outside.
Now as for electric a 20 gallon tank probably would do the job perfectly for you, if you have a shower and like to take long showers maybe go with a standard 40 gallon unit. Its funny the 20 gallon units save lots of space but I noticed in many cases they cost as much as a 40 Gallon. You just have to have a 220 V line ran to it. The advantage is ease of install, and cost. disadvantage is your using electricity to heat the water which in most cases can cost more than a NG unit, and slower recovery time if you run the unit out of the hot water.
I do hope this helps a bit.