Credits can be more difficult to retain if you don't renew, but if you only have a $10 credit they would be foolish to risk losing your monthly revenue over the sake of $10. Also, offering to renew would put you in a position to negotiate a better retentions plan, while going month to month really wouldn't. Your currently plan might be tough to get any further credits on anyway. If you aren't interested in lower monthly fee or if they won't go any lower, I don't see any benefit (except for case the won't renew your current credit without a contract renewal). If a renewal is required to keep the credit, you could always consider a 1 year renewal.
Unlocked iPhone 5, unlocked iPhone 4
Unlimited airtime, Unlimited CAN/US long distance, Unlimited SMS to CAN/US wireless numbers
2500 Call Forwarding minutes to CAN/US numbers
Google Voice for visual voice mail with message transcription, conditional greetings, unlimited messages (vs 35 message cap), remote retrieval from any PC or phone, no auto-purge after 10 days and most importantly no $7-$8 charge.