ProRes can be played on both Mac and PC. Quicktime supports playback on both.
For file size H.264 .mp4 works in Quicktime and WIndows Media Player 12 but that's not for a post workflow. All modern professional NLEs I know of can play Apple ProRes though.
If it's preview clips for the client, I would suggest just h.264 in the MOV container. Small, cross platform and quickly provided.
If you're using ProRes, I assume you're bringing it through the entire pipeline as an intermediate. If your clients are savvy or are taking over the post process at some point then it's a safe choice as it's highly supported across video programs both editing and finishing -- well, Windows is decode only.