There are two “~Sou” grammars in Japanese. I am going to assume that you are speaking about the “~sou” that you use when you see something, and not the hearsay/report “~sou”. The “~sou” grammar you’re comparing with “~rashii” is based on the speaker’s simple guess, and also what he/she sees. You could use this when you see delicious food being displayed or someone eating it in front of you but you haven’t tasted it yet. In that case, you could say, “おいしそうだ。” This “~sou” is used mainly for any kind of visual impressions you have.

I hope this helps. This explanation is based off my sensei’s notes. I will be studying Japanese for the third year now in college. Good luck with your studies!