I chose priority since my Uni told me that the average time for Tier 4 right now is 10-15 days. Unfortunately, the holiday weekend is here, so I will not be able to give you a clear idea of exactly how long the priority takes, but I overnighted my application yesterday, they sent me an email stating that they received it today and that since it is priority I will get emails updating me on the progress (one when it is being reviewed by an Entry Clearance Officer and another when a decision has been made) and that it typically takes 48 hours or so if everything goes smoothly. Exceptionally, it will take longer than 48 hours. I think it will be worth it given what panda said- it's a busy time for Tier 4 applications and I want to purchase a plane ticket ASAP- two weeks can make a difference in ticket price.