Which is more popular, baseball or soccer?
Which is popular, baseball or soccer?も同じ意味として相手には伝わりますが、
最初にあげた例文ですとどちらも人気だけどどちらの方がより〜というニュアンスになります。
また、補足ですが選択肢がふたつ以上あって何が一番と聞きたい場合は
What's the most popular sport in the U.S?
アメリカでは何のスポーツが最も人気がありますか？
となります。
Which is bigger in the States, baseball or soccer?
こんにちは。
スラング的なカジュアルな表現ですが、big は「人気」という意味もあります。
例：
Anime is big in France.
フランスでアニメは大人気だ。
Which is bigger in the States, baseball or soccer?
アメリカでは野球とサッカーどっちが人気なの？
ぜひ参考にしてください。
Baseball and soccer are examples of sport. The above expression is a common way to compare two things. If it is more than two you can say " Which sport is the most popular?"
"baseball" と "soccer" はどちらも「sport（スポーツ）」です。
上記の表現は二つの物を比較する時によく使われます。
もし三つ以上のものについて言うなら、" Which sport is the most popular?" と言えます。
Between baseball and soccer,which one is more prominent?
*You also need to clarify whether you are asking about popularity of these sports in a specific country or location.
"Which sports is more favored?Baseball or soccer?"
You asking which sports is enjoyed/liked by the people in that city/town/country.
"Between baseball and soccer,which one is more prominent?"
You are asking which of the two sports gains the most popularity and enjoyed or known by the people in that place.
この場合、特定の国や地域での話なのかを表す必要があるでしょう。
"Which sports is more favored?Baseball or soccer?"
その地域、国でどちらのスポーツが人々に好かれているか尋ねる場合
"Between baseball and soccer,which one is more prominent?"
その場所でどちらのスポーツが人気をより博しているのかを聞く場合。
You can use either of the above to ask this question.
The most = the superlative form of asking about something.
Who's the most famous man in the world?
"Bill Gates."
More = we use this construction when comparing 2 things.
"John is more professional than Peter."
上のどちらも使うことができます。
the most: 尋ねるときの最上級
Who's the most famous man in the world? 誰が地上で一番有名かな？
"Bill Gates."ビルゲイツかな。
More：ふたつのことを比べるときに使います。
"John is more professional than Peter."
ジョンはピーターよりプロフェッショナルだよ。
Which sport is more popular, baseball or soccer (football)?
Is baseball or soccer more popular in your country?
If you want to ask which sport in more popular in someone's country you would ask, "Is baseball or soccer more popular in your country?" Or you could say, "Which is more popular in your country, baseball or soccer?" Either way works. Using the word "Which" helps someone understand you are asking a question, but the first sentence is a little more common.
To generally ask about sports you could ask, "Which sport is more popular, baseball or soccer (football)?"
相手の国でどちらのスポーツが人気があるのかを聞くときのフレーズです。
"Is baseball or soccer more popular in your country?"
（野球とサッカーではどちらが人気がありますか？）
"Which is more popular in your country, baseball or soccer?"
（あなたの国では、野球とサッカーどちらが人気がありますか？）
どちらも使えるフレーズです。"Which" は、相手に質問をしていることを分かってもらえますが、はじめの文の方がよく使われます。
一般的にスポーツのことを聞くには、"Which sport is more popular, baseball or soccer (football)?"（野球とサッカーではどちらが人気がありますか？）と言います。
Which sport is favored more where you live? Baseball or soccer?
Is baseball or soccer more mainstream where you live?
Both of these ways are good expressions to use when asking about the popularity of two sports. “Favored” and “mainstream” are both synonyms for the word “popular”. This means they can be used instead of. Knowing synonyms is a great way to “spice” up your English conversation. Other words that can be used are lovable, preferred, beloved, praised or pleasing.
日本 
アメリカ合衆国 
南アフリカ 
イギリス 
