You can use either one, the only issue to be concerned with is the possibility of oxidation due to too much headspace in the secondary but IMO if your secondary time is only for a short time it's not a huge concern. Long term conditioning is a bigger problem.
I ferment 6.25 gallon batches in my 6.5 gallon buckets and when I do secondary I then rack to a 6 gallon better bottle.
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