Assuming your table is indexed correctly you will likely not see a difference. Per @mark's comment, use whichever one suites your needs.
The logic of which one to use should be your main concern.
If you're only trying to see IF there is one, then using field
makes the most sense, since it's limit 1 and only returns a single field.
If you want to know how many there are, then you'll need count
.
And if you want to know IF there is one, and retrieve it's data, then first
or exists
is the way to go.