Question about dual core CPU

Im in the process of setting everything up to play h.265 files with a new video card I got. (Nvidia 1030) what I'm trying to figure out is which is the better CPU to pair it with, using what I have.

Athlon x64 5600+
Or
Sempron 140

Both are going in a MSI k9n6pgm2-v2, with 4gb DDR 2 on what will be Windows 10 x64.

I guess what I'm trying to ask is if any of the codecs actually take advantage of the dual core part of the cpu?

The video card will do all the heavy decoding work. The CPU will be used for audio and subtitles.

The Sempron might be the better choice as that has the best single core performance. Subtitle rendering is single threaded.