Don't let go the domain to expire, if you think it have decent value. Always renew before expiry dates, so registrar does not charges you high later.
Also there is no gurantee you will get the domain name again, there may be registrar auction, expired auction and backorders. So securing the domain at first is the key to not loose sale. All research must have to be done before acquisition or registration, so it does not create any confusion during renewal time.