〜そうだ (伝聞) vs 〜そう (様態)
Hearsay 〜そうだ attaches to dictionary or plain forms to report secondhand information learned from someone else or the media. Conjectural 〜そうだ attaches to verb and adjective stems to describe an immediate prediction or impression based directly on visual appearance. Swapping them changes the claim from relaying what you heard from an external source to stating what something looks like right now.
〜そうだ (伝聞) I hear that …
明日は一日中雨が降るそうです。
この店のケーキはとても美味しいそうです。
〜そう (様態) looks like, seems about to
空が暗くなって、今にも雨が降りそうですね。
このケーキ、甘くて美味しそうですね。