I suppose it depends on whether we're talking about the lab exam or about real-life deployment!
The CCIE Voice lab has very little to do with R&S as Avner mentioned. The topics that are covered are really not the same as they are in CCIE R&S (there's no Cat6k in the R&S lab). So there's no real benefit or tie-in to do both in terms of one helping you prepare for the other.
In real-life work though, I find that a lot of people doing voice deployments don't understand enough about the underlying architecture or seeing any potential problems during deployment. Most of the voice networks I am brought in to troubleshoot end up not being "voice" problems, but more likely bad QoS design, or bad/unknown spanning-tree design such that the packet flow isn't what's desired.
The CCIE Voice exam, though, doesn't really focus on issues such as this.
PS. Best of luck in your preparation for either track, and certainly keep in mind that both Avner and I work for (competing) CCIE training partners, so always take what we say with a grain of salt, but we should both be honest with you! :)