I bet you haven't actually tried EX. :) but good point on VC vs. STP. biggest challenge in large scale data center is to find a way to get rid of stp. cisco came up with vss for 6500 and vpc for n7k/n5k. juniper came up with vc, force10 seems go for vc too. haven't tried running vc at juniper EX8k data center core - only at access layer so far, but N7k's vpc at core seems running well. not sure how good THRILL would be in future.