The pre-filled carts may be a good way to test different flavors, but if you get 5 pre-filled carts for $2.99 and you can get a six pack of 5 ml samplers for $15.99 you see that the bottles are a much better deal. Sure you will most likely end up with a few bottles of flavors that you don't like, but hopefully what you do like will make it a better bargain. Find a few that you like and order the full-sized six pack next time. Refilling your own carts may be a little less convenient, but it is way less expensive once you know what flavors you like.
Of course this is coming from a guy who has a 15 ml bottle of 555 which is a flavor that I don't care for at all, but I feel like I got my money's worth out of the other 5 bottles in that six pack.