The differences in the between Shorin-Ryu, Goju-Ryu, and Shotokan are, in the grand scheme of things, going to be relatively small. There's going to be a different set of kata for each, and some different details on stance-work, etc, and some slightly different emphases in terms of strategy and principles and mechanics. But in the grand scheme of things, the differences we're talking about are like comparing Spanish and Portuguese. Yes they're different languages, but compared to Arabic or Chinese, they're still extremely similar. It's going to feel much more familiar going from Shotokan to Shorin-Ryu or Goju-Ryu than, say, going from Shotokan to Olympic Taekwondo, or from Shotokan to BJJ.
Both Shorin-Ryu and Goju-Ryu are Okinawan Karate, and so a key difference between them and Shotokan is the Kendo influence in Shotokan (the heavy emphasis on longer-distance strikes and one-shot-one-kill) is not as present in the Okinawan styles. They tend to emphasize a bit more closer range, a bit more circular techniques. But like I said, we're talking about a shift in emphasis more than a completely different set of mechanics and philosophy.
