From what I hear they're kinda similar (at least in principle). Like a lot of people who play upright also know bass guitar, and vice versa. I guess it's just a matter of learning to get used to the different length necks, but from what one of my bass-friends told me playing an upright bass is no harder than playing a fretless bass guitar (and this guy learnt on guitar so I'd trust his word on it).
bass guitar is ok tho