Headphone choice is very much dependent on what purpose they will fill, e.g. pleasure listening or mastering. Grado's fit the bill for pleasure listening but are not very helpful for mastering as the extremes are difficult to discern leaving you with momma or poppa bear, which we all want baby bear! Senn HD 280 are better for mixing, due to them being closed ear and assisting you with low end monitoring but they are muddy, not the best resolving headset available and do not cut it for pleasure. They are great for on location, though. ATM 50's are a budget scream and cut the middle well. A bit of pleasure and monitoring here but ultimately too many compromises to be real enjoyable pleasure cans but better than the HD280's. AKG 701 series hit it down the middle, but again, the low end is tough to monitor and are more for pleasure than mastering. They also don't have the dramatic impact I desire in cans. And, yes, I've driven them with all kinds of amps.
Good in ears can give you the best of the two, good mastering and pleasure, as long as they fit properly. If they don't they are terrible and will be extremely colored. Generally, they require a very good seal. Believe it of not, Thinksound MS1's are inexpensive and beat pretty much all of the above. IMO and I've been through all of the above. That being said, I'm sure other in ears have great potential and should be explored.
HD 600 and 650's are an excellent choice and cut the two well, however, I found that Hifiman 400's are the best of both worlds, pleasure and mastering. I would not buy the Hifiman 300 series due to some bad reviews and they are not planars.
Now for some other considerations, but ones I have not listen to, Audeze, Oppo, Mr. Speakers, Focal BE and AudioQuest. Stax might also be a option. All of theses are much more pricey but, I have extreme interest in them, too.
You might notice that many of these are planar type cans. My experience with cans have led me in this direction as they seem to do way more right across the board than other types.
Finally, a word about Grado. I have owned the 60's (2 pair), 225 and and RS 2's. The fit can be problematic for some and the RS2's were the best sound and fit for me. However, until they have a complete redesign I would probably never buy another pair of Grado's as their signature lays in the middle and do not cover a broad enough spectrum for either pleasure or mastering. If you are all about midrange, at the expense of bass and treble, well these are for you. However, I prefer a more balance set of cans. If used for mixing you will never really know what is happening at the extremes and can over or under do it in the mastering process.
In the end, for mastering, I have found that having several sets of cans is best, as well as a good two channel stereo set up. This way you can compare, compare and compare so that you get it right. But if you are limited and want cans for pleasure and mastering I would recommend Hifiman 400 or 500 series. But, please, do get out and try the others I mentioned but have no experience with and report back.