Balancing with MX records - How does this work?
The MX record preference is used when more than one MX record is entered for any single domain name that is using more than one mail server. In this case the preference number indicates the order in which the mail servers should be used. This enables the use of primary and backup mail servers.
So if you want to load balance you should give the records equal priority.
You can find more information here about load balancing emails.