Talk:Warframes/@comment-79.95.152.12-20130406121612/@comment-7700557-20130406165210

Steel Fiber increases Armor: it only takes effect AFTER your shields are depleted. Redirection increases total shields. Fast deflection increases the speed at which your shields recharge. Take into account that they all increase by a % of the base value, so it's generally better to increase what you have the most of already; also, that shields get passive boosts with your warframe's rank, and Armor doesn't.

Increase armor if you already have a lot of it (Rhino) and if you expect your shields to be drained very quickly (if your shields are always up, Armor will have no use). Personally I prefer increasing shields, but I'm a noob so take with a pinch of salt.