Well the pro's of the R4 is that it's really reliable,
there's not been a firmware update lately because
it's almost perfect emulation wise, it's simple to use, and the cheat.dat is the most compatible with it.
I have't played the CycloDS, I do know that it is similar to the R4 and it is about a tie between the two, however the CycloDS is still having firmware updates, in the future it may surpass the R4 alot or maybe not.
Either way, I prefer the R4, it's main point is reliability and that's what I want in a flashcart.
I am not an expert, if you want to know the full stats then check reviews on GBATemp or an expert site on flashcarts,
hope I helped

Edit: My post may be a bit biased. Here's some unbiased parts- If the R4 doesn't get updates eventually then the flashcart may become pretty bad, there's a rumour that the R4 team has quit and the R4 may decline... I personally think that some people would make updates for the R4 in the future who aren't the R4 team because it's quite popular but that's a maybe... So far it's pretty efficient though...