I'd go for the timber building every time. It'll be warmer and just nicer.
Only trouble is when the insurance company asks were you keep your bikes. They prefer a concrete garage as they don't usually burn. So thinking of some sort of flame retardant inside would be a good idea. Plaster board and a skim would do, (ceiling as well.) Or two layers of plasterboard. Obviously puts the cost up.
Regarding the base; I prefer to not have a pad at all but rather build a base wall maybe only 2 bricks high and then plant the timber building on that. I'd then fit floor joists on jiffy hangers fixed to the base wall and floor with 22mm moisture resistant chipboard.
Before flooring insulate between the joists level to their tops with the thickest insulation you can budget for.
Make the slope up to the doors from outside as long and shallow as you can. It's hard to push a bike uphill!
Steer well clear of up and over doors. Roller doors are marginally better but you can't beat a pair on hinges with good tie backs for windy days.
Go with LED lights from the start and don't forget to put a nice heavy cable in for your Electric bike charging point..
