I have previously owned both pair of headphones so I will offer my opinion of them both.
In terms of sound signature, the Grado's are very forward and "in your face". The highs are very clear but can be harsh at times. However I find that mostly dependent on what song you're listening to. The mids are very good and vocals can sound very warm though not as full as other headphones. The bass is very tight and punchy though they lack impact. It's very polite and doesn't overpower the music but can be lacking for certain genres. They have insanely good detail for their price and are especially good for anything that uses a guitar (rock music, pop). However, this doesn't mean that they are bad with other genres of music. However, I do find them lacking in hip-hop and certain songs for jazz where a bit more impact and extension is needed for the bass. Also the soundstage is not very wide. They do leak noise but not that much depending on the level of volume you are listening at.
I found the Grado's to be very uncomfortable since I have large ears meaning that the bowl pads that they come stock with really dug in. However, upon changing the pads to HD414 pads (the big yellow ones) and quarter modding them, they offered similar sound quality (a bit worse, bit more bass) to the bowl pads. The build of the headphone itself is not the best that I have seen and they seem a bit fragile. However, they are well built and as long as you don't drop them multiple times or sit of them they should last you a while.
For the V-Moda, the sound signature I would consider it a very balanced sound though the mids were a tad bit recessed than I usually like. The highs are pretty good but not as good as the Grado's. They are definitely more tame but they lack extension in some areas. The mids are pretty good as well and I would say they are slightly worse than that of the Grados in terms of quality. The bass definitely kicks ass though. They completely blow the Grado's out of the water in terms of impact while still maintaining clarity. If you like bass in your songs, then definitely go with the M80's as they add a lot to the listening experience. The M80's do well with all types of genres of music so they are very versatile.
As for comfort, they were to me about the same in comfort. You can change the pads for the Grado's to get different levels of comfort and the V-Moda's come with nice leather memory foam pads. I had to return the V-Moda's after two weeks even though I really enjoyed the sound because they were too uncomfortable for portable use. They would hurt my ears after 1-2 hours and I couldn't really justify their purchase since I already have a nice pair of IEM's in the Etymotic HF5's. However, many people have said that they take time before you get the pads to conform to your ears and then they can be worn for many many hours. The build of the V-Moda is vastly superior. They are built like a tank and they seem like they will last a long time. You can probably abuse them as much as you want and they would still be OK.
In summary, the Grado's definitely offer better clarity and detail. The sound is also more natural since they are open headphones, you don't get the "vacuum" effect of your ears feeling congested when listening to them that I get when listening to IEM's or closed headphones. If you want a more balanced sound with less clarity (but still very very good) and more bass, then go with the V-Moda's. Comfort levels about the same for each for my big ears.
Enjoy your day!
Edit: Forgot to mention that I can wear my Grado's for about 4-5 hours before I need to take a break with the new pads. And noise isolation from the V-Moda isn't the best for a closed can.
