Developer groups like paranoid android, cyanogen or aokp have a single repository of their code which is then merged to each device's device tree - so all kitkat devices supported by cm have the same features.
Where as sure, its easier to get stable plain aosp with NO ADDITIONS on a nexus device, that in no way guarantees that the additions cm etc add will be stable. So in answer to your question, more directly, there's no difference.
That said, the device tree for the s4 may be problematic as samsung aren't particularly forthcoming with their code.
If you're choosing a phone based on a wish to use open sourced based roms, go for the nexus
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I do NOT reply to support queries over PM. Please keep support queries to the Q&A section, so that others may benefit