I was wondering if there is any clear rule as when to use plain form vs の
eg. その食べっている人。 ラメンをたべるのが好き。
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They are grammatically different. ご飯を食べている人 is "person who is having a meal". This phrase ご飯を食べている modifies 人.
Nominalization with の is used for nominalizing a phrase or sentence. This phrase ラーメンをたべる is nominalized by using の such as ラーメンをたべるの.
For example, ラーメンを食べる人が好き(I like the person who eats ramen), ラーメンを食べるのが好き(I like eating ramen).