I often use both, or used to but my little bike puter got smashed and a crash and I haven't bothered to replace.... I do like the instint read out of the little computers on the bars (i'll never put a multi hundred dollar unit on my bars.. asking it to get smashed or stolen.. or lost)
I do find wheel based read out to be much more accurate in the woods that any GPS. at least for Speed/distance anyway.
got to get a new one.
also found the cheapo wired ones are better than the fancy over prices wireless units. oh and lighter too...