M1 - You are a current 737 driver, so you should know if you can hack 5 years SH or not.
2006 is to be a year of high recruitment, so you will lose a lot of seniority by waiting. With the likely changes to retirement ages later this year, by waiting you could find yourself last man in, and stuck at the bottom for years.
If you only joined BA for LH, then I guess it may be worth the wait. If you want the career, then I would join ASAP.
(PS - Starting on 777 2nd Feb - weekends in Newark beat months in Basrah any day!!)