Because of how powerful Nocturne is in general, both roles are fabulous for him. However, due to him being a poor duelist (Generally), you need to watch your match ups to make sure your opponent doesn't counter you. There was a FOTM which saw Nunu in top lane for a while, Nocturne excelled during this. Mostly because Nunu was all but completely crippled if a Nocturne player could consistently block incoming Iceblasts. However the list of Nocturne's winning match ups ends there. He lacks the harassment of most champions because he can't deal much damage outside melee, and his outright killing potential is mostly wasted against other bruisers because he doesn't have the finishing potential to rake in kills on his own.
This means that Nocturne is a better ganker than a laner, his gap-closing and clear times are nearly unmatched except by a few other champions; (Namely Lee Sin and Shyvana), combined with fairly powerful CC, make Nocturne one of the best junglers and gankers. He's fallen from his former glory over time. He was extremely powerful during the first two seasons. But even now, he's a strong assassin who can disorient entire teams and take out enemy carries extremely fast when built glass cannon. Or he can play as a bruiser... Well, nevermind that, he's a pathetic bruiser. There once was another FOTM that had him building Frozen Mallet, Sunfire and Wit's End (WHICH WAS HILARIOUS). The importance of health has been emphasized in season 3, so he's a lot more durable now. But he's still similar to Kha'zix and uses a very similar glass-cannon play style and build. When you build, look for the following...
-Jungling speed through Hunters machete or lane mobility with boots and potions with both.
-Damage and utility though a Brutalizer, or Phage if you've failed your first few ganks or otherwise fallen behind.
-If you're doing well, pick up the pace with a pair of Berserker's Grieves, or hunker down with Ninja Tabi, if you're having trouble with shielding stuns or powerful slows, grab a Mercury Treads, but only if CC is really giving you trouble.
-Toss in Wriggle's Lantern here if you've Jungling, sell it later, when you build another lifesteal item. Make sure you have Tier 2 boots of some kind at this point.
-Get a little bit of health through things like Phage if you don't have it already, then finish off Youmuu's Ghostblade, and work on Frozen Mallet with a Giant's belt, or go straight into another Brutalizer if you don't need the extra health.
-After you have a Phage and Ghostblade (With or without a Giants Belt), get another Brutalizer, and either finish Frozen Mallet or skip it and go straight for a Black Cleaver. Nocturne is very sticky and hard to peel with just Ghostblade's active and Duskbringer.
-Get damage and sustain through Bloodthirster or Ravenous Hydra. Note that these synergize well with each other, but make sure you mix in defense with your damage and lifesteal, or enemies will burst you down. Only get both if you can drive home a win easily.
-If you still have a Phage, work on finishing off Frozen Mallet at this point.
-Balance out your build with things like Guardian Angel or Atma's Impaler and Maw of Malmortius for sustain or durability, once again, depending on how well you're doing.
(Just a Note, this is a little confusing, but there's no rush on the Mallet unless enemies are carrying an unusually large number of Summoner Ghosts or other movespeed steroids and you cannot compete with the raw movement speed and therfore need a slow. Or, if you are dying a lot and you need the extra health instead of damage.)
As for tactics, Nocturne is very much a right click to win champion. Make sure you land Duskbringer, or at least set them up so you can use them to chase enemies even if you don't hit them. Make sure you block incoming disables with Shroud of Darkness, only block damage if it is an extremely powerful nuke or if it attaches a debuff which can disable you, such as Brand's Sear ability. Always use Unspeakable horror on the enemy you intend to kill, or an enemy you wish to run away from if a gank or dive goes sour.
Finally, once team fights start, you win want to fight with LIFO tactics, which means Last In, First Out, the opposite of a tank or bruiser. You are very squishy as an assassin, which means you want to make sure your tank is in position to make himself a target, usually this is right after they initiate. Allow your team to whittle down enemies, or get them out of position, then activate Paranoia, and dive in after vulnerable enemies. Make sure enemies do not target you, or disable you in such a way that you cannot escape with a flash or by having your tank bail you out.
Your job is to quickly nuke down squishy enemies who would otherwise turn the tides of a battle for their team. And snowball the fight from there. You want to attack isolated enemies, who cannot be saved by their tanks or supports by the time you kill them. Usually this is the AD carry, but be aware of the enemy team and what their roles are, and who would most benefit your team by being dead.
-GLHF and stuff. Feel free to ask more questions if you like.