Repeating what other people said, but I've done both Big Cheese and Rodent Pro. (I have only one snake, but I usually make large purchases with 3 or 4 of my snake owning friends to split shipping costs).

In terms of packaging, Big Cheese is great. They are all vacuum packed on a styrofoam tray. My order came completely frozen (although I ordered in the middle of December). Mice were all consistent in size and very clean. For me, the packaging and overall product trumped Rodent Pro. Their stuff came frozen for me, but were packed in large bags full of air, which I thought would eventually compromise the freshness of the mice. They're okay still right now (it's been about two months) but I have my doubts on whether or not it'll stay nice and unfreezer burned. Both the mice and the rats I ordered were consistent in size and mine were on sale (whee!). I guess in terms of pricing, Rodent Pro might be a little cheaper. Customer service was great for both companies.

I had another friend who orders from Layne Labs. They are a little more expensive, but from what I hear, a good product.