Since these are kind of similar, Kanto appearing in both, being on the same handheld etc. Which did you prefer? These two sets of games are commonly known as the nostalgic games so it would be interesting to see what people preferred and why.
I personally preferred G/S/C since it was an expansion to the originals and I really enjoyed the storyline throughout Johto and to top it off going to Kanto to get another eight badges was awesome.
I'd have to go with G/S/C. It would kind of be more interesting to play G/S/C first, and then be like
"Wow, there was actually a game of what happened a few years back?" R/B/Y wasn't as interesting as their sequels.