Differences between 2 and 3 inch?

[SaggyG]

Hey all,
I'm posing this question because I had basically designed my still. 15
gallon keg, 5500w with control, 36 inch by 2 inch column with a 12 inch
boka on top, I have height restrictions. Well today at my work Christmas
party I won some money and can put it towards my build. I have basically
ignored the 3 inch posts because I was going 2 and wanted to keep my focus
on what I was going with. I was going to order my parts this weekend but
now am wondering if the upgrade to 3 inch would be worth it. And if so
should I stay with a 2 inch head on a 3 inch column or just make it all 3
inch?
Thoughts, suggestions, links all welcome and appreciated.
Saggy

P. S.
Thanks halfbaked!