As a rule of thumb, primary swap space would be on a swap partition.
If you use an non-journaled file system then you run the risk of having a fragmented swap file. A swap file is usually used if temporary swap space is needed. Why?

When swaping to a swap file it first must decide where to write, in order to do that you have to:
-Find out where the file is.
-Find the correct cluster that the blocks belong in.
-Write the files there, BUT
-If it's bigger than one cluster it has to find the next cluster.
-Then it write in that cluster.
-It will continue writing to and finding clusters until it finishes.

There is one advantage to a swap file however. Should you wanted to increase your swap size you can do it without changing the partition table.

With a swap partition there's less work involved. A swap partition basically writes wherever it wants to. Should you have two or more drives I always put a swap partition on a seperate disk, that way you can reduce seek times. Additionally a swap partition placed on the outside of the disk is faster due to the increased speed found on the outside edge of the disk. If you use imaging software for example or compile programs it could make a big difference but if you just do general day to day things it might not be a noticeable improvement.

But that's just my 2 cents worth.