They are both about the same on my Michigan lawn. If I had to pick one I'd go with SBM first.

I've seen better results with the SBM, although the greenup seems to take a little longer. Milorganite advertises it contains iron (5%, I think) but it didn't seem to make a difference.

I'd recommend SBM for the nitrogen, and greensand for iron.

In time you'll get noticable results with both. Use which ever is cheaper and easiest for you to find.