A bit of both. For some people, I have such a firm idea of what I want to give them that I search high and low for it (I have that creepy Cancer/INFJ habit of remembering off-hand comments of what people like - example, my boyfriend tried on a knitted Sock Monkey hat and he remarked that he liked it, so he's getting that for Christmas). Others, it really can be a Wild Card of looking around and seeing what "feels right" to give them.
When we're shopping for family members, or each other, my partner and I always have a list for the one big present we buy and then we're spontaneous for the smaller stocking stuffers we buy. It works out well for us
Both, if Mrs doesn't know what to get for someone she wanders and looks until she gets an idea. She never purchases though. That is only done with a list. This drives Mr, as in Mr Spontaneous, crazy. He only puts up with it because he gets great sex
most of the time I have a pretty good idea as to what I will be getting for each person, but I dont take a list. I also will change my mind if I find something that I feel will work better for them then my original idea.