I listen especially to rock, post rock, some metal, and rap.
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If you like any amount of bass I would get the Sennheisers. The grados are bass-light headphones. Personally I HATE the comfort of Grados in general, the Sennheisers are going to allow you to listen for much londger periods of time without discomfort.
Both of these cans are open. Are you OK with open cans (they leak sound and don't isolate well)?
Beyond that these two have very different sound signatures. The on ear Grado is forward, in your face, and presents music as though you are sitting on the stage with the band. The new 80e has more bass than 80i--to the point that I would not pick one over the other on bass or lack thereof.
The over ear 558 is velvet smooth and presents a much richer sound. Most people find it to be the more comfortable.
I'd recommend trying them both out at a store (or ordering both and returning the ones you don't like). I really wanted to like Grados, but within about 30 minutes of use they make me nauseous. My (fairly sensitive) ears don't like something about how the driver is sitting; it's practically directly on your ear. It actually took me a few days to realise that this was the problem. At first I thought I was coming down with a cold, but then I found other people posting about the same issue. It's the only headphone I've ever used that did this to me.
You really need to try them both. They are different is so many ways. There are many who swear by Grados and find the Senns to be relatively boring and unengaging. Comfort can be an issue with both, but is more often a problem with the Grado.
I like both. I don't have a comfort issue with either. Your mileage on both issues may vary.