I've used Big Cheese once, and CBC once, both for small rats.
Big Cheese's small rats were probably more consistently the same size- My CBC small rats definitely range from the smaller side of small to the larger end. If you have multiple snakes who might need slightly different sizes, it's actually quite helpful. I think Big Cheese's shipping was a bit faster, but my CBC rodents still arrived very frozen, and were packaged very well. They aren't vacuum sealed, but if you want to sort them out by size and vacuum seal them yourself, that's not a big deal.
Even with the variation in sizes, I've been more impressed with the quality of the rodents from my CBC order. My Big Cheese rodents were a little dirtier than I cared for. My CBC rodents are not only quite a bit cleaner, but also plumper, even if they are a little smaller. Horatio is a big fan!