Rod of Ages gives you a very tidy sum of stats compared with what you pay for it, you just have to make sure you complete the item itself not too late in the game since it takes a total of 15 minutes for Rod of Ages to max out its stats. It is an all round good item and the life it gives can make up for a few mistakes you may make while playing.
Archangels Staff however fullfills a different role. Rather than an all round caster item, Archangels is the item you buy to counter Kassadins inevitable mana issues. It gives a huge amount of Mana and Mana-per-5 and the AP it gives via its passive is not to be shrugged off either. Of course you can get blue buff to help counter your spells sucking away your mana, but play a couple of games with the staff and you'll find you can spam even more than you usually do, which is a good bonus for Kassadin.
If you find either you're not finishing Rod of Ages early, or you're not making too many miststakes, I personally would reccomend the Archangels Staff. Rod of Ages is amazing and the Catalyst gives you lots of laning power but I think the staff will get you further if you're comfortable with controlling Kassadin and feel you don't need the bonus life.