～そうです

You are airing your mattress outside. With which sentence will you run out to get the mattress in?
a. あめが ふりそうです。
b. あめが ふるそうです。

Both sentences are conveying the meaning that it is not raining yet but you have a reason to believe it may rain soon. Then what is the difference?

It is who is making the call.

あめが ふりそうです using the [verb stem] is when it is you who think it is going to rain, probably by the look of the sky or from your arthritis pain.
あめが ふるそうです using the [plain form verb] is when you hear somebody else say it is likely to rain. You are not personally convinced yet.

So the answer should be a.

Grammar Points

• [verb/adjective stem] + そう – You yourself think that something is about to happen or something looks/sounds/smells/feels/tastes ….
• [verb/adjective plain form] + そう – You are repeating what you have heard.

Examples with verbs

たいふうが きそうです。It feels like a typhoon is coming.
たいふうが くるそうです。 I hear that a typhoon is coming.

あたらしい おみせが できそうです。It looks like a new show is coming here.
あたらしい おみせが できるそうです。 I hear that a new show is coming here.

Examples with いadjectives

あの ケーキは おいしそうです。[stem]
That cake looks/smells/sounds delicious.
あの ケーキは おいしいそうです。[plain form]
I hear that cake is delicious.

あの せんせいは きびしそうです。 That teacher looks/sounds strict.
あの せんせいは きびしいそうです。 I hear that that teacher is strict.

この えいがは おもしろそうです。 This movie sounds entertaining.
この えいがは おもしろいそうです。 I hear this movie was entertaining.

Examples with なadjectives

あの テストは かんたんそうです。[stem]
That test seems to be easy.
あの テストは かんたんだそうです。[stem + だ]
I hear that test is easy.

この みせの やさいは しんせんそうです。 The vegetables in this shop look fresh.
この みせの やさいは しんせんだそうです。I hear that the vegetable in this shop are fresh.

あの せんせいは しんせつそうです。 That teacher looks/sounds kind.
あの せんせいは しんせつだそうです。 I hear that that teacher is kind.