While I was learning I used a stash of cloth pads, which are also handy for the tail end of my period as liners.
Some people react to the natural rubber in some of the other cups, which is what swayed me to the Mooncup with its medical grade silicone.
Also, just speaking for the Mooncup, the stem should be cut to a size you're comfortable with. There are only two sizes - pre-childbirth and post-childbirth, making it easy to choose which one to get.
I looked into the Keeper, not sure if it's still available, and there were too many reports of allergies, so I gave it a miss.