When VCO operates at high frequency, clock signal transition time remains comparatively low and that results in low jitter. General practice is to operate VCO at highest possible frequency and divide it down to required frequency. Up to some division factor there will be significant improvement in jitter till other source of jitter becomes substantial contributor. As on chip parasitic are much less, there won’t be noticeable increment in radiation when VCO is operating at higher frequency. But pin as well as board parasitic plays vital role in radiation, higher frequency output will have higher radiation compare to lower frequency output.