I'm not much of a Tudor fan and a big TAG fan, but if you are talking about a 25500TN-0001, there are a few advantages to the Pelagos. First, it's a Titanium case which is a big plus in my book, much easier to heft. The Pelagos uses an MT5612 movement (one of the Kenessi variants) and is a Certified Chronometer. The Tudor version of the Kenessi movement has a silicon hairspring so by-by to magnetization (I have a Breitling Superocean Heritage with the B20 version of the same movement with a metal hairspring and it's currently magnetized).
The TAG uses a bog plain ETA 2824-2, not a Certified Chronometer. The MT5612 has a seventy hour reserve where the 2824-2 is only thirty-nine.
A couple thoughts in TAGs corner; Titanium is easier to scratch and shows it much more boldly, any ETA movement is much easier to service and parts are available everywhere.
I wouldn't worry about the size, they both look fine. The TAG's only a mm larger.
I personally don't care for the snowflake hour hand but it is easy to see. Oh, and I can't stand that clown laughing at me.
Good luck, you'll be fine with either choice.