[eluser]Phil Sturgeon[/eluser]
Its a tricky one as you could easily use either. I often start off writing a helper file then convert it to a library as it grows.
I preffer to use a library for a series of functions that are often used in sequence or use shared data between them (i.e if it needs to return more than one value, or you are passing it lots of params to each function).
An important thing to remember is with libraries you get flexibilty. With the library name you get namespaces, emaning you dont have to worry what they are called as it wont be redefining an existing function in another helper.
I can see there being much of a performance hit. The tiniest bit slower as its loading a class instead of a page full of functions but... I really cant see it making any noticeable difference no matter how many people are blasting away on your site.
