LCMS/MS versus Western Blot
Western blot, though is a commonly used technique and is relatively simple to do, has some issues:


Low throughput: it is difficult to analyse multiple proteins simultaneously
Limited cross comparability: since antibodies to different proteins can have different affinities, they cannot be compared with each other.
Low sensitivity
Not very quantitative


LCMS addresses all the above limitations of western blot. It is also possible to do targeted proteomics i.e. study just some few proteins instead of the entire proteome.
An LCMS system has a high initial setting up cost but a relatively lower run cost. However, the mass spectrometer has to be maintained and it is not an easy job; you basically need a facility and a dedicated technical staff.


  Which one would you recommend and would you do both for validation?


Which technique to use depends on what you really want to see and what resources you have at hand. It is a commonly practised approach to do a western blot for re-validating a few selected genes identified from LCMS experiment, which is just to prove that the result can be obtained using another technique too.