Morde VS Nasus Bot
So why not take Nasus instead of Morde bot? I realize that Morde gets the dragon bonus but let me break down both kits for you:
Nasus:
P: Lifesteal, good for sustain - rather than taking up an ability slot, it's a passive. It can get to the point that he can ignore people if he is ahead. If you built even one lifesteal item on Nasus it gets ridiculous. Q: Nuke, scale to infinite - you're a juggernaut essentially that when you do reach your target - you will absolutely massacre them. W: Slow, a slow that scales so hardcore that all you have to do is apply it to enemy ADC in lane and call for a gank - easy kill. Or kill the support that lacks a dash. Overall, this ability is much more powerful IMO than Mordes ability. E: AOE AP Damage. It's meh, you really don't wanna use it in lane otherwise you'll push. R: AOE AP Damage - regularly procs and gives a major boost in HP
Morde:
P: Shield that scales with your HP. Makes you tankier, sort of, it'll pretty much pop after like 1 quarter of an ability or half an auto attack from an ADC. P: Dragon gimmick. Gimmick is very gimmicky, because it's a gimmick. Q: Nuke that gradually hit harder. Doesn't scale infinitely. W: Movement speed, sustain, damage that is very depending. Allies can use it to escape or move in quicker. E: AOE Hit, consider it useless later on into the game other than waveclear, giving you a shield that will decay when you dance around a lane or jungle for it to decay. R: AOE DOT - Kill somebody, take their soul. This is an impossibly strong ability that sucks the absolute life out of the power budget that morde holds. Put the power budget right up along with the dragon and MOST of your power budget is based around the ultimate and the passive abilities.
Both of these champions are weak against:

Both of these champions excel at playing against:

Both of these champions work well with ally support:

Both champions lack the abiltity to compete against marksman - I am not saying that Morde or Nasus are meta or even 'good' picks, let's be honest here - both of those picks are absolutely awful. BUT.
If you were to pick Nasus or Morde? I'd pick Nasus.