The Gleim course is the easiest course. The FAA requirement is 16 hours to complete. American Flyers requires you to be online for 16 hours the Gleim course should take you less than 2 hours. Also they used to give Expressjet employees a discount. I think it was 100 bucks for the renewal and the paperwork processing. Just call them and give them your employee number. You will not be able to get the discount if you order it online.
As far as a FSDO renewal it all depends on the office you go to. You must be a 121 captain. Some will renew and others will not. Call them ahead of time to find out if they will do the renewal.
Lots of great responses, but I just finished renewing mine so I thought I'd add. I've used Gleim for all three of my renewals and had nothing but great experiences. Easy to complete and killer customer service. I'm a lazy SOB, so I spring for their ACR service as well.
If you do American flyers make sure to print out the tests or save to pdf with your answers in the boxes. You can then use them next time around.
If your CFI exprires, you have to take another ride with the FAA, or a DPE if you can find one that is allowed to do CFI's. Best bet is to use the one you got it from the first time...even better is to go out and get another rating and tag on the CFI to the ride (seaplane, ATP, etc). It resets the clock for all CFI ratings.
General rule is NEVER let it expire. I like AOPA's Jepp course the best, no time constraint and easy test questions. Finished in 2 hours.