The preferred way of provisioning VMs through vRA is to use the built-in IaaS composite blueprints. XaaS is not designed to deploy VMs but rather to deploy "other things". The pros of using IaaS blueprints include built-in reservations and actions which specifically take into consideration virtual machines. Cons would be that you are somewhat (**maybe**) limited by this system as it has been built. By contrast, XaaS blueprint pros are that you can be much more flexible and deploy essentially any way you want. However, severe cons include you having to recreate the wheel for all of this logic. This in turn means it becomes custom code which you write, debug, maintain, and, most importantly, support.
Bottom line: Unless you have extremely specific and unusual requirements which you have proven that IaaS composite blueprints cannot support, you should always use IaaS blueprints.
