Seq.iter vs for - what difference?

As others mentioned, there are some differences (iter supports non-generic IEnumerator and you can mutate mutable values in for). These are sometimes important differences, but most of the times you can freely choose which one to use.

I generally prefer for (if there is a language construct, why not use it?). The cases where iter looks nicer are when you have a function that you need to call (e.g. using partial application):

// I would write this:
strings |> Seq.iter (printfn "%c")

// instead of:
for s in strings do printfn "%c" s

Similarly, using iter is nicer if you use it at the end of some processing pipeline:

// I would write this:
inputs |> Seq.filter (fun x -> x > 0)
       |> Seq.iter (fun x -> foo x)

// instead of:
let filtered = inputs |> Seq.filter (fun x -> x > 0)
for x in filtered do foo x