IMO the thing which can make your life really a lot easier is to keep a cluster (relatively) homogenous. Mixing different CPU models from the same vendor can be OK, at least if you use a controlled CPU type for VMs which has the same capabilities on all host CPUs, e.g., "QEMU64" or "KVM64".
Mixing vendors in the same cluster works for some, but there are surely more problems comparing to clusters where all CPUs come from the same vendor. While the instruction set accepted is both amd64, they are architecturally so different that it may still making issues on live migration, or live snapshots, even when virtualizing.
As said, can work, but has bigger chances to make problems compared to more homogenous clusters.
If that does not matters much, it depends a bit if you need more cores for the bucks or slightly more single-core performance.
For the first I'd personally go for AMD for the latter maybe Intel, but it's hard to get CPUs with small nanometer fabrication nodes from intel, so maybe even there you would fare with AMD at least as good as with Intel.
Security wise AMD has (currently) also a much better track record, so there's that - if that's important depends on how much you trust the virtual guests (i.e., are they controlled by external people, maybe even not employed or known to you?)
So in short, it depends. What guest operating system and what services do you plan to run there?