Is there any relationship between tikz circuit library and circuitikz, for example that one is build on top of the other. Are they compatible in some way? Is one of them in some way obsolete?
The TikZ circuit library is inspired by circuitikz. From the TikZ manual (Chapter 29: Circuit Libraries):
Written and documented by Till Tantau, and Mark Wibrow. Inspired by the work of Massimo Redaelli.
Massimo Redaelli is the author of the circuitikz package. Apparently (see @CountZero's comment) the two are not compatible, However, as far as I know there is nothing the circuitikz package does that the TikZ circuit library can't. So I would just stick with that.