Considerations of buying vs letting a space are probably the same as for buying/letting a house. Renting is money you'll never see again, but you don't need to have the capital up front, whereas if you buy then the value could increase or at least stay the same. In London at least parking space prices seem to follow house prices to some extent.
We have an off street space with our flat and it is very useful when you need to get things to and from the car. Knowing that we can always park near to our door is good as well. We used to have a flat where we parked on the street, and you could guarantee that on the day it was raining and you had lots to carry then you'd end up parking miles away.