This is actually my current home headphone selection. IMHO I would keep both cans (as I did) and use them for different things.
The Sennheisers get a lot of use when I listen to classical music and anything that would feel at home in an opera hall/theater. They have a more lush sound compared to the Grados and, as has been mentioned, more accurate and spatial soundstage. This works well for classical pieces and larger scale vocal performances (opera for instance). These headphones are actually quite bad at certain kinds of music which would b out of place in the aforementioned concert hall. This music includes pretty much any jazz, some female vocal stuff and a lot of rock and other guitar heavy music.
For the stuff that sounds odd (not bad, just odd) on the Sennheisers the Grados step up and fill the void nicely. They are more in your face and there's not much soundstage to speak of, but they put you right in the middle of what I would describe as an intimate, cafe-type setting. I heavily prefer these cans when listening to Eric Clapton or Carlos Santana as the crisper presentation of the music lets me get a sense of the musician's bond with their instrument, something which is staggeringly important and often overlooked in the presentation of music. This aspect of the music takes it to a whole new level which, I feel, justifies my rather substantial investment in this hobby. It's not that same as attending a live performance, but the extra level of detail brings me one step closer to feeling like I'm sitting in on a jam session with these guys (which would be awesome BTW).
Another class that I might suggest (I apologize to your wallet BTW, it has to be said) is a set of well fitting mid- to high-end IEMs. I'm currently listening to UE studio fi 3s, but will be upgrading (probably to customs) this summer. These will allow you to catch every minute detail your music has to offer. I was listening to some Norah Jones today on my way to class and noticed a very faint pizzicato violin accompaniment in the background of one of the songs that really brought a sense of unity and polish to the song. It's certainly not necessary, but these levels of detail will carry over into your listening of your other cans and raise the level of enjoyment just that much more. Plus, they're nice to have when you travel.