I use 12x12 more often, but I do use my 12x24 too. The 12x12 for regular use. The 12x24 for party decorations, treat/gift bags and large projects -- tags for 3 kids for disney autographs, class projects, ribbons for swim team etc.
I look for a good sale with free shipping on-line. Last year, I ordered a year's worth from custom crops.com in November. THis year only the 12x12 mats were on sale, so i will get my 12x24 mats at J's with a coupon.
THe problem for me has been the 12x24 paper. I can't find it in my J's, coupons are excluded at M's and ACM so I honestly don't know if they carry it anymore. It wasn't on sale at custom crops so I didn't order it. OhMyCrafts.com has a great selection of 12x24 paper but the shipping is OUTRAGEOUS!!! I keep checking for a free shipping sale, but it never seems to be in conjunction with the paper sale.