I would look at what you want to with the switch, and what is important to you.
For example if you don't need the stacking, don't need redundant power supplies, etc then the EX3200 may be better than the EX4200. There are some features of the Juniper EX that don't line up perfectly on the Brocade CES 2000 switches.
In some instances the feature-set of one vendor may be better for you than another.
In other instances a provider may simply wish to standardize on one platform from one vendor. This generally is acceptable for distribution switches, etc. However on the Edge / Border and firewalls you may wish to use two vendors to reduce and hopefully eliminate targeted attacks to vendor vulnerabilities. For example if a Brocade or Cisco exploit surfaces you may still have a Juniper router on the edge that is "safe" from that attack.
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