When to get Rabbadon's vs Luden's

Luden's is much better late game, Rabs before Luden.

It's not a substitute item, any mage will likely always want to have Rabadon's. Luden's is a replacement for DFG.

[{quoted}](name=Yulen Xoshxius,realm=NA,application-id=WEuoGbmp,discussion-id=8EeakkAu,comment-id=,timestamp=2015-03-13T00:36:48.653+0000)

Any advice on when to get which over the other?

It depends on what mage you're playing. If you have high dps/short cooldowns (karthus/orianna/ziggs) or high mobility (katarina/leblanc), you may get it first over rabadon's deathcap. I highly recommend it as your 1st/2nd item because it gives you a big power spike early game once you buy this item. It gives you 100 EXTRA magic damage (thats dmg outside of your skillkit) on up to 4 targets with the passive. Who wouldn't want this kind of benefit?

Get both, I usually get Rabadon's as a first 3 items because it's a hella big AP boost especially early game.

Neither. Get void staff for single target.

Luden's Echo is, however, good on certain poke champs(nidalee), as well as people that cast a lot(Cassiopeia). Get it if you need wave clear + AoE, or better poke.

Seriously though. Void staff is amazing. get it more