I have not seen a specific recommendation, and in a cluster of all 207's it should work fine. What I see happen in practice is that an AP with the highest performance headroom (i.e. the least utilized AP, or an AP with higher CPN/memory) is selected as the preferred master.
Also, it may be useful to check in the Aruba Instant VRD to see what features are utilized in your environment. Things to take into consideration are a lot of RADIUS proxy, DHCP server, NAT (magic VLAN). If your design depends heavily on those features, it may make sense to design with a higher capacity AP as the VC.
If you are in doubt, you probably don't need the higher capacity AP. And in my personal opinion, it is most times better to have all equal (architecture) APs in a cluster. That avoids multi-architecture upgrades. Also, if you decide to put a 325 in as a virtual controller, you probably need a second one for redundancy.
Maybe others on this forum have examples where they needed dedicated virtual controllers.