You are correct that there are two subscription levels, but the Portable Subscription does not support streaming. It only supports downloading Performer Store music to your portable device...AND...it turns out that not ALL devices are supported. I found out the hard way -- by paying the higher subscription fee for a couple of months -- that my Razr Maxx is not supported.
In any case, for the Portable Subscription to be worth even an extra nickel in subscription fees, it needs to support BOTH downloading AND streaming of Performer Store music on BOTH Android and iOS. It boggles my mind that this not already the case, and I am extremely frustrated that no one at JRiver seems to care, or is willing to explain why this limitation exists.
I have posted multiple times on the Performer Media Center forum - that's what the Performer Store used to be called - and all I hear are chirping crickets, so it's pretty clear that the Performer Store is an afterthought. I think that's a real shame because MC is hands down the best media player and organizer in the market, and the Spotify player STINKS. On the other hand, Spotify makes it super easy to download and stream music to a portable device. Seems like the easiest part of the problem for JRiver to solve.