I can only offer my own experience as a 5'4 girl (I have a 29" inseam to your 30). My Santa Cruz--a 26er--is more comparable to the XS Salsa than the S. I think I'd feel too stretched out on the S Salsa. But hopefully the shop and Salsa can help you navigate this decision. How does the geometry on your current 26er compare? If I recall, that bike is too small for you.
And in what way did you feel cramped on the XS Salsa that your friend owns? Without knowing how she has the bike set up, it's hard to know for certain whether the frame itself was too blame. For instance, if she's running a short stem and has her seat forward, I can see why you might feel cramped.
Do you have any idea whether your femurs are long/short relative to your total leg length? This is the issue that I have that dictates a lot in terms of frame size and bike set up. My legs are short, but my femurs are relatively long. For most bikes to work for me, I have to use an offset seat post and put the saddle fairly far back on the rails. Once I do that, it effectively lengthens my reach to the bars. It make sense, generally, for me to get the smallest frame that I can otherwise make work for me; otherwise, the reach is too long once my saddle is in the right position. If you have this same issue, then I would probably get the XS.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher