Which of the following is more appropriate / polite?
I would like to bring a couple of facts (or things?) to your notice. OR I would like to let you know a couple of facts.
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Whenever you are writing to someone it's really important to use words that sound polite. So it would be better sounding to the reader if you use "I would like to bring a couple of facts to your kind notice/attention".
I agree with check123. "Facts" can have a harsh, accusatory tone which could put the listener on the defensive. "Things" is usually a vague word to avoid, especially in writing, but it can be an acceptable way of toning down "facts," if the speaker wants to avoid sounding belligerent.
Yet there are other ways to accomplish the same thing. You could avoid the word "you" as well:
Let me state the facts...
Please allow me to give the facts...
also sound a bit more polite, and less confrontational.
While speaking it may well depend upon your tone. In writing, let you know a couple of facts, sounds more authoritative/accusative compared to bring to your notice.
Alternatively, bring to your attention, may be appropriate when dealing with seniors in the hierarchy.
One of the single word for the above phrase can be accentuate.