I've got the packers, my daughter has an REI clone of the roller style. Either works well. As Doug notes, it boils down to personal choice -- hard to debate (sensibly).

The roller style may be slightly more waterproof (if you're planning to swim across rivers, etc.). It's a bit more fiddly to get closed, especially when a bit overfull. The packers are easy to close, but sag if the pannier is lightly loaded.