I think either way can be valid, it depends largely on the response time you need. In the past I've done systems that "cross over" from counting to timing at a certain point to keep the display updating at a reasonable rate.
At 50Hz I think you are in a grey area unless you are happy with 10-second or worse display updates. With a 1-second update you would have a 2% jitter.
That said the best way (easiest to get an accurate result) would be to count for a long(ish) period.
Rob