Bottom line: RNA injection into the cytoplasm is the most efficient way for generating both efficient mutagenesis and numbers of live pups. However, plasmid DNA works too. For me, the main concern with RNA is the reliability of the RNA synthesis to provide consistency across injections.
Sci Rep. 2014 Mar 28;4:4513.
Validation of microinjection methods for generating knockout mice by CRISPR/Cas-mediated genome engineering.
Horii T, Arai Y, Yamazaki M, Morita S, Kimura M, Itoh M, Abe Y, Hatada I.