honestly that is for you to decide. i can list my thoughts... but it would be my own opinion and may not be the same as yours.
i prefer speakers as you can likely tell from my build pics http
/postimg.org/gallery/ql0xj0nu/ for positional audio. i really dont like wearing headphones unless i have to (i bought my m50s to use when traveling and also while at work). i hate headsets and the only headphones i would use are my studio headphones except for earbuds i use while working out (since they are cheap and replaceable).
personally... i'll never give up my surround sound set and studio headphones.
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razer surround + your earbuds you have now: its cheap.. and if you're happy with how razer surround works go for it. you wont get the best sound out of your current earbuds but if its good enough for your needs then you can save some cash and perhaps buy something really nice later on.
razer surround + new headphones:
the trouble is that your soundcard may not support good headphones because it might lack enough power to drive them. you can likely power something like the m50's though. they arent the best for soundstage but you will get a whole helluva lot of increase in sound quality if your card can power them.
new DH soundcard + your earbuds you have now: likely a waste. while dolby headphone is toted to be good i would pair it up with half decent headphones to make best use of it.
new DH soundcard + new headphones: if you have a soundcard capable of DH plus its capable of powering good headphones then you have a great solution. paired up with something like a hd598 its about the best you're going to get for headphones unless you spend quite a bit of money. even if you just use a pair like the m50s or dt770 its going to sound better than a headset.
surround sound headset: if you're looking for easy plug and play or relatively inexpensive headphones that have surround then you might want this route. they have a mic included so you dont have to worry about buying one. sound quality is typically significantly less than good headphones but so is the price. you also dont normally need a soundcard for headsets.
new soundcard + SS headset: unless you buy a set which has 3x3.5mm inputs then its a waste of money. usb headsets have their own soundcard inside so they bypass any soundcard which makes it useless.
new soundcard + speakers: this is the most comfortable option capable of the most boomy bass. if you arent worried about waking up or bothering others its great. you get true surround sound as well. speakers dont always play nice with microphones but you can get them to work without echo if your placement and settings are right.
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however thats just my take on things. i'm sure there are people who think just the opposite (and quite a few people love headsets).
pick whatever will fit your needs best.