45mm is more popular, the best selling, and great for general use for all hand sizes. It is also the industrial standard for pole dance competitions.
50mm is a wider diameter, the traditional size and usually harder to grip for most.
They have come out with 40mm poles now that are skinnier and easier to grip, especially for those with smaller hands.
I use a 45mm in my studio and at home.
Here is an article that will help you decide what pole size is best for you >>
If you are looking for a “specific” cross comparison between 45mm vs 50mm go here >>