Heads up: I'm an incoming first-year at Oberlin, and my investigation of Grinnell was pretty cursory, so my knowledge of both schools is limited. That said, some general comments...
Be aware that Oberlin is nearly twice Grinnell's size – a good thing or a bad thing, depending on what you prefer. I was worried that Grinnell's size and location would be isolating, and found that Oberlin struck a perfect balance: a great sense of community without being claustrophobically small, a rural town 35 miles from Cleveland. On the other hand, I know people who visited Oberlin and decided that 2,800 students was too big. So it's worth some thought, especially if you can't visit.
Oberlin offers more academic programs than Grinnell; if you're interested in supplementing your English coursework with Creative Writing or Cinema Studies (or taking courses in archaeology, dance, environmental studies, geology, international studies, Italian, neuroscience, women's studies, among other things) you won't find anywhere near the same breadth or depth of offerings at Grinnell. We also have a significantly larger library, a top-ranked art museum (and the Art Rental program!), and brand new, top-of-the-line centers for science and environmental studies – not to mention programs like Winter Term, the Experimental College, and the co-op system. And as a function of our location and the presence of the Conservatory, it's easier for us to bring renowned and diverse events to campus: over 400 musical performances a year, not including other speakers and presenters.
So, my bias is obvious
That said, they're both extraordinary schools, with a lot more similarities than differences – you really can't go wrong with either one.