VMware Upgrade from 4.1 to current?

Hi,

We are looking to upgrade our VMware infrastructure.

• 3 ESX Hosts running VSphere 4  (4.1.0, 800380)
• 1 VCenter Server running Version 4.1.0, 925676)
• All is SAN attached

Some questions:

1. Should i upgrade to the current version of VSphere? 5.5 or 5.1
2. What is the upgrade steps, in what order do i upgrade? the systems are live.
3. I read somewhere the datastores can not be upgraded? does this mean i will have to create new Datastores migrate the VM's to this?

edit:

I found this link: does the same apply if going to 5.5?

How to: Upgrade to vSphere 5 VMGuru

Thanks