I was looking into these capacitors:

1. https://www.digikey.com/product-detail/en/R76TI23304040J/399-12611-ND/5731102/?itemSeq=300867881
2. https://www.digikey.com/product-detail/en/kemet/C1210C223KFRACTU/399-10471-1-ND/4172923

Which one would perform better for DC filtering at the output of a high voltage three phase rectifier?

 

Normally these values are used across high microfarad capacity smoothing devices in order to decouple any high frequency.
What is your intention for them?
Max.

 

I intend to use one of these high voltage capacitors for high frequency decoupling at the output of a three phase rectifier which has 600 VAC line to line. Ripple is not an issue, so I'm not using any smoothing or electrolytic capacitors. Which one would do better, a film cap or MLCC?

 

Although the 360Hz ripple will be low, what are you using unfiltered rectified 600v for that requires HF decoupling?
Polypropylene caps (PP) are usually used in high frequency apps. Such as SMPS power supplies,oscillator circuits and audio equipment etc.
I have also used WIMA MKP4 for HF applications.
Max.