I think I am going to have to reassert, and to agree with most, that it's all about context. Fundamentally, any two planets are part of a general resonance of the whole, so how much either influences the other and in what way is entirely contextual. To establish a separate rule, except a grammatical one that the faster applies to the slower and should be phrased that way, is artificial. So, it always goes both ways, but one may be more (or rather, differently) favored if it is more strongly placed than another. That's not only in a fixed chart, but in the context that any chart is a slice of an evolving resonance and therefore some planets are growing in strength (by progression, if nothing else), and vice-versa.
Whether you consider Mercury and Venus special for their lack of broader aspects depends on whether you stick to geocentric. If you don't, then retrogrades are meaningless (don't happen in helio), but for most they are quite meaningful. Therefore the interior position of Mercury and Venus do indeed make them special.
But in any case, asking what one planet does what to another out of context is akin to asking whether E is a major third to C or is C a minor sixth to E. By themselves, they are either and both, but functionally are neither until you establish what key you are in or what chord you are playing -- and what the last and next harmonic changes involved are. It's at least that subtle, and in fact the parallel is very close, as planetary frequencies and resonances tend to fall out in the same proportions as the musical scale, the divisions of light, and other scaled subsets of resonance, see http://www.astrococktail.com/rainbows.html