Playing 0314 RKCB bidding 4C has it's dangers. But since we're playing 1430 it seems sensible to agree the suit we want to play in slam rather than confuse partner with 3H and then later trying to sign off in 6C perhaps.
I'd be pulling to 6♦, 5♠ was no doubt fit showing in some way, or partner misunderstood Ghestem (which happens 105% of the time in my experience). If the director gets called I'll leave it up to him to decide if my 6♦ was allowed.
I voted diamond since it'll be hard to justify to partner why I didn't lead his diamond void when he has one. Though my intuition tells me that [for the same reason(s) as dburn] that a club might both be best and create a match swing (hopefully for us).
I'm afraid that the BBO Flash window seems to be pretty much unusable now from Ubuntu 7.10 in Firefox. It used to work perfectly but not any more. Maybe I've missed a bug comment somewhere but I get logged out on average every 20 seconds or so with error #2031 :-(