i5 versus SM57
As part of my degree I did experiments investigating exactly this and after a long time I found........not that much difference, quality wise at least. It is true that they sound completely different but that depends on what you prefer, just personal taste. My personal taste was for the i5, however, I seem to find that when the "standard" microphone comes up against an Audix then I prefer the Audix but that's just me. For example, I would choose a D6 over the AKG D112 when close micing a kick drum. The only way to truly decide between them is just to rent/borrow/find one of each, try them out and see which one you prefer.
If you have the money get both and then if you're ever in the situation where you need the "sound" that the other mic gives then you're sorted, but life is rarely that kind.
My personal taste - the i5 - you'll just have to try both for yourself.