BIND will round robin the MX records, how mail agents deal with
multiple MX records of the same priority is less of a known factor (to
me at least). Actually I'd be interested to hear from people who try
both configurations for say a month at a time to see what influence each
configuration has if any.
RFC 974 reads:
In the third example, consider a mailer on A.EXAMPLE.ORG trying to deliver a
message to D.EXAMPLE.ORG. In this case there are only two MX RRs, both with
the same preference value. Either MX will accept messages for D.EXAMPLE.ORG.
The mailer should try one MX first (which one is up to the mailer, though
D.EXAMPLE.ORG seems most reasonable), and if that delivery fails should try
the other MX (e.g. C.EXAMPLE.ORG).