Is it worth building Rylai's instead of RoA on Cho'Gath for the on hit 40% slow with his E (Vorpal Spikes is a spell effect not an on hit effect, so it procs) or is the eventual better AP, Catalys sustain and extra HP just too good to pass up?
On my opinion Rod of Ages is still better due to the mana, if you don't have an huge mana pool you will easily become OOM as Cho'Gath because his abilities have incredible mana costs
"Why not both?"
need mana or slow? simple question
Both is great with abyssal and zhonyas you be am tanky and above great damage slows not stacking hurts but if you aa someone they can't run outside dashes then when they dash pop up cuz they lost their escape
I've been thinking about this myself on quite a few champions. Because the HP on RoA was lowered to 500 at max stacks (10 minutes), I feel like it might be better to get Liandry's or Rylai's on champions who could make do without the mana as they give comparable AP and have stronger combat passives, but I feel Cho'gath needs the mana enough that RoA is warranted, though if you can get a blue buff or a Frozen Heart early on, I think getting Rylai's would be better.
Personally I've been looking at AP Gragas with this kind of question; though he's kinda weak right now. With recent buffs to his Mana at early levels, I feel like you can get away with skipping RoA and going straight for Haunting Guise into Liandry's. His base damages are high (and scalings average at best) so getting Pen early would actually benefit him. The difference in AP is negligible as the Penetration you gain and the Passive on Liandry's would most likely outweigh the 20 AP you get after 10 minutes with the item (though you get more AP when you buy the item). The loss of 200HP is kinda harmful for his ability to go all in by midgame, so I have reservations. I figure with double doran's into Haunting Guise, he could manage his mana in lane well enough until he could buy a Sheen into either Lichbane or IBG, and then he would be set, but I haven't tried it out quite yet; hp sustain on Gragas isn't really a problem so he doesn't benefit as much from the Catalyst passive other than for the mana, which is why mana sustain is the only real issue. 
core and any of these items to finish the build: 
This is basically the build I've used on him but swapping RoA out for Liandry's.
I do think that Rylai's is a good item right now on AP champs for a semi defensive build who either don't have heavy mana issues or not a lot of spammable CC. Gragas doesn't like Rylai's because his Stun and slow would make the slow useless, but I think Chogath does for his scream and vorpal spikes to have extra slowing power for him to better land his knockup. If it were me, I would try going for Morellonomicon into Rylai's as Cho'gath likes CDR and it is an earlier offensive powerspike with 80AP over 60AP when you buy the item at a lower cost 2300 vs 2700. When you force Cho to sit on the Catalyst, he is similar in Gragas in that he only really benefits from the mana sustain as his passive keeps him healthy, so getting a cheaper Forbidden Idol + AP, he can be more of a threat to his lane opponent without losing much in terms of lane sustain. He can then get Lucidity boots for (near) max CDR and focus on his other items or you can take CDR/lvl Runes and get any boot you really feel like: Tabi, Mercs, speedy boots, or Sorcs. You can then focus on your big AP items like D-Cap or focus on defensive items if you need to be tankier for your team.
