If you're going there to pick them up and have a few hours to spare, take the bus. But I would not recommend taking the bus back. As I'm sure you're aware, buses are quite prone to pickpocketing. When that bus goes through province, I would say that having huge suitcases would make pretty nice targets. It's doubtful that anything would happen, but I'm sure the several colectiveros who have had their fingers cut off by thieves thought the same thing.