Major difference between BIG-IP GTM and LTM Physical and Virtual Appliance

I think the biggest difference between physical and virtual is what's often called "speeds and feeds" - the amount and throughput capacity of the appliance. While the newer VE licensing can blur that line a bit, the software is identical, so you're basically missing the hardware-specific acceleration features of the physical box (SSL, caching, compression, etc.). One other consideration is the box itself. The platform is ICSA-certified, so you can put a BIG-IP in your DMZ and treat it like a firewall (with or without AFM). If you run it in a VE, there's an additional hypervisor layer above the BIG-IP that is now dependent on the security of the virtualization vendor and the commodity hardware you're running it on. Considering that a GTM is usually located closer to the edge of the network than other load balancing services, it may make more sense to go physical.