Stuk,
In regard to 3"1/8... What's your height and weight...?
And what length have you selected?
Firstly, stuff resale... If you want 3"1/8 just do it!!
A 9'6 Fireball (EvoI) x 3" is going to be enough foam for most people...
But 3"1/8 will give you extra waves...!!!

It depends on your paddling strength too... paddling ability... how often you surf... if you think you are a little bit down on any of those areas, then 3"1/8 might be for you... especially if you are 90+kg, or 95+kg. If you are 80kg or less, you only need 2"7/8 to 3".... A wet winter wetsuit/steamer is also up to 4kg...