LTM and F5 DNS on same hardware box or using LTM and F5 DNS on different hardware/vcmp guests

Given a choice and no financial concerns, I prefer to run dns and application load balancing on separate devices, *and* license LTM+DNS on the DNS host.  If they're going to share the same virtualization platform, it has to be sized to accomodate DDOS capacity. 

There's a couple advantages of running LTM+GTM as a package - mostly with L7 irules, and load balancing options for your pools of DNS servers behind the Big-IP DNS system. It's not something that comes up often, but every once in a while, those LTM L7 iRules will be able to save the day against a (D)DOS attack against your dns servers. For running the two concurrently, If they are the only two modules you'll ever run, and you're in a condensed environment, it works well. 

If you intend to run more modules on your LTM - AFW, APM, ASM, etc, you'll find that having the DNS separate from the load balancing will provide you with a cleaner interface, and room to stack more relevant L7 modules on the load balancing device. 

If you are in a high DDOS risk environment, having the DNS separate from the application services will be a best practice, so you can use the DNS system to monitor and mitigate the attack, and move applications between your datacenters.