Megan,
Congrats on your permit for the JMT!
Going to be an amazing trip for you.
I did it back in 2011 over 15 days.
That said….what is your resupply plan?
Knowing how many days of food you will be carrying at any given time would determine how large a canister you may need.
If you are going South Bound from Yosemite to Mt. Whitney, you have a few places you can cache food/re-supply.
Tuolumne Meadows….we left food in a bear box at a car campsite close to the Post Office, so that we only needed 1.5 days of food climbing out of the Valley to the Meadow.
Then you have Reds Meadow, where you can mail yourself a resupply bucket.
Then you can go to either Vermillion Valley Resort or likely Muir Trail Ranch, which is effectively your last easy resupply for the next 110 miles to Mt. Whitney.
Options after MTR are to leave the trail to hike out at some point that I don't recall to resupply or something like having a pack mule meet you on the trail with additional supplies.
For our group, we carried all of our food from MTR to Mt. Whitney.
I was using a Bear Vault and had a tough time fitting 6 days of food in there and ended up cheating by carrying that day's food outside the bear canister.
Because of my experience on the JMT, I later ordered a custom 12" tall Bearikade (it is the same height as my bear vault, but holds more because it is wider in diamter).
The 12" Bearikade gives me 750 cu inches vs.the 10" tall Weekender which provides you 656 cu inches of storage.
http://www.wild-ideas.net/custom-bearikades/
The Bear Vault BV500 provide 700 cu inches.
http://www.bearvault.com/bearvault_details.php
Based on my own experience, I felt having 750 Cu Inches would make it easier to get 7 days of food in the canister.
Curiously, when I called Bearikade, the man who answered said that he already had a "custom made" 12" tall Bearikade sitting on the shelf, ready to ship.
He also mentioned that the 12" tall canister was a commonly requested size.
The Expedition is much larger than the Weekender and might be more than you need.
Also, bear in mind that if you are traveling with others and you have an Expedition, your resupply constraint will be the size/capacity of your hiking buddies if they have a smaller canister than you do.
Anyway, point is that you don't have to be limited to "either/or" the Weekender or the Expedition.
Given how expensive they are, getting a custom cut one is reasonable in cost and easy to do.
If you are interested in checking out mine….I am in the East Bay and could meet you at the Walnut Creek BART station.
Also, I will tell you that I recently acquired this canister and have not taken it out on a trip yet, but I do feel that the volume is going to work for me based on my JMT experience.
Tony