Can anyone just give me advice on when to buy either of these? I know the differences between them but say, for example, when playing cass I always go RoA despite that she has a lot of range and should not really need all the extra HP ( and i see most people buying this as well) So, if you could please just give me a rundown of when to buy which item it would be appreciated! And which should i be buying on karthus as a side note? lol Thanks!
On 
, I usually get both 
and 
. He needs tons of mana to keep up with his costs; 
and other mana regeneration items simply can't keep up with his E in the late game, plus CDR isn't very good on him (it's a noob trap for lowering his ultimate's cooldown; his Q and E hardly scale with CDR at all).
It is possible to get by with either one (I'd suggest Seraph's for more mana and damage, plus the shield... But you'll be missing out on more damage and E time by not buying supplementary mana), but they synergize extremely well.
You are correct in saying that it takes a lot of time to charge both RoA and Seraph's, but they give Karthus some extremely valuable tools and almost unparalleled DPS when combined with pure-damage AP items (
, 
, 
). He needs a strong foundation to make up for his squishiness and his mana-hungry E, and RoA+Seraph's don't hold him back (they provide damage, health, and mana). In Karthy's early game, you can rely on your Q's single target proccing for all the damage you need while you complete your 
and RoA.
Many people will tell you to replace Rod of Ages with 
. Personally, I get both. My favorite Karthus build is 
(for CC reduction and a little MR), (tons of base stats, Catalyst passive for lane safety, and mana synergy with Seraph's), (max mana, plenty of damage, and a little extra survivability from the shield), (tons of health and damage, synergy with your Q's single target procs), 
(magic penetration, more health and damage, synergy with Rylai's and your Q's single target proccing), and one of the 120 AP items or Void Staff (depending on whether the enemy team is a significant threat to my life, has a lot of AD, and/or has a lot of MR).
It's a tankier build than your typical tons-of-AP Karthus, but living longer means more Qs and more E ticks, and Liandry's+Rylai's ensure that even the tankiest of targets doesn't want to sit in your E.
why not both?
Usually you get RoA on more of a tanky caster or someone who will be spamming a lot. So Ryze works, Maokai works. You can also use RoA if you feel like you need to be a little tankier, say they have a hard diving / gap closing comp. It stacks up in 10 minutes I believe, so doesn't take too long to get fully charged as an early item. However, if you feel you can stay relatively safe and are a burst mage / nuke (brand for example), you may just want to go for more immediately hard hitting items like DeathCap.
Seraph's is usually either considered kind of core on a champ, such as ryze, or is used as a decent sustain item early, into a survival tool later. Tear only costs 700gold, so you can start stacking it early and get a Seraph when you feel the shield would be useful. It is also very good if you are going to be spamming and burning through mana.
Both of these items in conjunction is definitely for late game power and sustain. You will be giving up early game pressure but you will get some decent gains mid - late game. However, to get them in time you do have to get the tear early, and then pretty much rush the RoA. This is either champion dependent (like Ryze will almost always do this) or enemy team dependent. If you feel like the game will not go late, then there is no need to rush these stats as you won't see the power benefits. This can also be said if the enemy team is crushing your towers early and you don't think you can hold out without buying power.
Remember to have an end game gold standard build in mind, but make adjustments for the game that you are in.
