I personally would go with a 2.5 mm. Links are smaller and it makes sizing easier. It is also easier to squeeze the links and remove/put on (JIMO. I have tiny hands).
Make sure to buy a quality prong collar, though. My last HS collar I bought directly from the HS site.
Some of the cheaper options available at retail stores can spring open while walking/training the dog and I have heard of people saying the pronged ends can be sharp.
+ Kalbi CGC 6/21/2015
+ Blackthorn's Out of the Frying Pan NW1 SCN BCAT SIN RATN TKN (Katsu) 9/12/2017
+ Blackthorn's Skofnung CGC TKN RATI RATO (Steel) 07/23/2018