Spell resistance isn't a good option for PCs. Yes, it protects you from hostile spells, but it also applies to friendly buffs and healing as well. Last thing you want is to be dying, and the clerics cure spell doesn't get through your spell resistance. Yes, you can voluntarily lower it to let a friend cast a spell on you, but it takes a standard action to do so, and also allows enemy spells to get through for a round.
As long as it doesn't grant any SR, you are good. SR is a bad option in most cases, as it can prevent your own side from aiding you in battle, or worse, while unconscious. Either they waste a spell on you, or they are forced into taking anti-SR feats and options they might not normally need.
To lower SR, it takes a standard action, which means no spell or no attack or half movement... which can often equal death. And if you are unconscious, you don't get a standard action to lower it.
However, the SR from armor is actually pretty good, depending on campaign. If you are sure to encounter a lot of lower level mooks with spells, then SR 19 might be great, especially with your CL 17-20 friends making the check on very low rolls. Otherwise, don't depend on SR. It is equally likely, perhaps more so, to get you killed than save you.