The mediums are becoming too small for me, and I am thinking that could slow down my progress. I am looking into moving up to the Ls. However, I am leaning more toward to the contoured ones, as my ribcage is on the small side. Not that I wouldn't consider the regular ones though.
That being said, I think you should take advantage of the offer, emmz. You will want them at some point and you may fill the mediums much sooner than you would have thought. It could take you a matter of weeks to move on to the next size, like DoubleD. Granted, everyone is different but it seems worth it to me.