That's a really general question so here's a general answer.
1) is a bad idea. Those usb drives don't have good write amplification management. Just the generation of log files will kill the flash. Buy more disks, spinning rust is fine.
2) has the highest performance when you can stripe your IOs. otherwise all of your vms are competing for io bandwidth on that single disk. You can get some better performance on a single disk with a read cache (rapiddisk and others) at the expense of system memory but that doesn't do anything for writes. writes require a io controller with a battery backed cache or a main memory that can crash to flash like NVDIMM.
If you want to really know the performance impact, measure it. Preferably with a workload that imitates what your users will do. iostat, blktrace and fio are your friends.