The tank build on Nasus is better suited to top lane, while the pressure build is better suited to mid lane. You can play either, however, depending on the situation.

Tank build should focus on farming up Q stacks, and stalling out the game if behind. The pressure build should win lane and be able to take towers incredibly quickly. The downside, however, is it falls off later, and doesn't provide significant DPS in drawn out fights.

Ghost is a viable summoner spell choice on Tank, and Ghost, Exhaust, Barrier, and Ignite all have situational viability on the pressure AP build.

On tank Nasus, building a Doran's Ring early and getting a few points in E before maxing Q is acceptable if you're expecting your tower to take a lot of pressure early. Otherwise, hold off on maxing it until second. If you're against a lane that can't push, there are cases when maxing W second over E are viable.

On tank Nasus, if you can afford to build to Triforce early without being run over, do so. Otherwise, build Kindle Gem for cdr, any additional tank stats you need to survive, and then Sheen into eventual Triforce, so you can at least work towards stacking Q.

Situational Item Notes:
Mercury's Treads: vs Overwhelming hard CC, and few auto-attackers ( Ninja Tabi are stupidly strong right now, so even vs heavy CC teams with lots of auto attack damage, they're still better)
Iceborn Gauntlet: If you need health + armor in lane, you can convert a Glacial Shroud + eventual Sheen into Iceborn Gauntlet later instead of building Trinity Force. This will lower your threat quite a bit, however, and shift you towards more of a supportive role.
Guardian Angel: Great defensive item, particularly vs magic heavy and burst heavy teams. Build instead of Randuin's Omen or Zz'Rot Portal.
Warmog's Armor: A decent alternative to Spirit Visage, for low magic threat teams, or as a stand-alone if you don't have 10% CDR runes. (Can also build Trinity Force + Frozen Heart in this case, and get your health items elsewhere).
Thornmail: Another great item against auto-attack heavy teams.
Boots of Swiftness: If you luck out and face off against a team with low hard CC, and low auto attack damage, these are your go-to chase-down boots.
Banshee's Veil: Build if you randomly need more Magic Resist and nothing else. More cost effective than Spirit Visage, but you lose out on the sustain and cooldown reduction. Good Magic Resist option if you went with the Frozen Heart build mentioned above.
Locket of the Iron Solari: A good item all-around if you find you're mostly fighting with your team, and even better if you're playing as more of a supportive tank role.
Banner of Command: An alternative to Iceborn Gauntlet, if you need Glacial Shroud armor for laning, but end up still building Trinity Force. Very good option for lane pressure mid to late game.
Youmuu's Ghostblade: If you just want to run over squishies, this is your only realistic damage option. Particularly good if you opted for some Lethality in runes. Not recommended in most cases.
Mortal Reminder: If the enemy tank is excessively armor heavy, and you need to play a bit more of a damage carry role late game, this is a fair option. Doubly good if you're against a sustain heavy laner such as Maokai, as you can opt for an early Executioner's Calling to shut down his healing.