UPSC or TNPSC, which is the best to choose?
What are the pros and cons of both?

I’ll answer it in details. Please avoid if it doesn’t fulfill your demand.
But I’m pretty sure it’ll work for you to come out of such dilemma.

First, let’s take TNPSC our decision to prepare for.

When you’ll start your preparation, going through the syllabus well then you may realize that it’s related to UPSC too in some senses. Slowly you may feel that I should prepare for UPSC syllabus first then after completing that I’ll complete PSC part in few months. Believe me, this happens with many aspiratns. So it will result in bigger dillema then. You may loose your track and can be messed in between. So to avoid such scenarios, you must be aware about futuristic things.
Your friends/ family members/relatives may suggest you that you’re deserving candidate for UPSC, you should try for that first and you may again loose the track. In India, we have this false image in the society that UPSC is biggest than any job in India.