The difference between relative clause and present participle as post modifier
For example,
" I have no idea why an interesting subject frequently affecting our everyday lives has suddenly become so vague and far away."
" I have no idea why an interesting subject which frequently affects our everyday lives has suddenly become so vague and far away."

Why is the second sentence better than the first one? Is there any grammatical mistakes or errors of any kind in the first sentence? Could you pls tell me on what occasions is it preferable to use relative clause instead of present participle as post modifier?

Question

The difference between relative clause and present participle as post modifier
For example, 
" I have no idea why an interesting subject frequently affecting our everyday lives has suddenly become so vague and far away."
" I have no idea why an interesting subject which frequently affects our everyday lives has suddenly become so vague and far away."

Why is the second sentence better than the first one? Is there any grammatical mistakes or errors of any kind in the first sentence? Could you pls tell me on what occasions is it preferable to use relative clause instead of present participle as post modifier?

Answer

1.  Perhaps because it's clearer.
2.  No, it's fine.
3.  You need to judge each case individually. You would choose one construction in preference to another on the grounds of neatness, clarity and lack of ambiguity.