Well budget is the key. If you are limited on budget, you can use ESXi (free version) otherwise, ESX 4 (Vsphere). More physical host or number of CPU sockets will cost you a lot more for licenses if you want to use ESX4. The advantage of ESX4 is you can join your host in VC, cluster so the administration and maintenance would be easier with Vmotion, Storage vmotion....
ESXi does not have luxury features but it works fine for all basic functionality. You can host as many servers as you want for no license fee. (you will see license fee costs you a lot). You can have more physical servers (hosts) if you want more performance. You just have to log on individually host's virtual center to manage your vms.
If you have more $$ you can run ESX4 and take advantage of all high availability features and clustering...
K