VSphere is by far the best, especially for bigger, resilient installations. HyperV is far cheaper, but not as mature, reliable or full featured.
That said, hold your horses for Server 2012 as by all accounts its far better than previous.
You're totally right about VMWares website, though.
Oh, and to be honest, if you're using ESXi without VSphere, then you're barely scratching the surface. Which is fine, by the way, but you do get a lot more functionality. Are you using a SAN, by the way?
XenServer is a bit of an odd one to me. We sometimes use it for Server Virtualisation, but we now primarily roll it out for Desktop Virtualisation (VDI) in which it does a very good job.
If it was a choice between current Gen Hyper-V and XenServer then I'd personally go for XenServer, but that's because I know it better than Hyper-V. I certainly don't think it's any worse than Hyper-V, but I'm not convinced it's objectively much better, either.
To say it has a few oddities would be an understatement, and the GUI is fairly limited meaning you very rapidly end up in the console whenever you want to do anything remotely exotic.
It's potentially less stable than Hyper-V, especially once you get into multi server (They call them "pools") deployments, but at the same time, it's also very rapid to deploy and fix so it's swings and roundabouts really. Also, this is purely anecdotal and we have may more XenServer customers than we do Hyper-V.
To be honest, the current generations (I'm excluding Hyper-V 2012 here) aren't a patch on VMWare VSphere and so if cost is lower down the priority list than most other things you almost certainly want VSphere. Other than that, take your pick based on cost/features/local expertise and accept the niggles!
That all said, I think VMWare need to watch their backs because everything else is getting better and VSphere is shockingly expensive still! I'm really looking forward to getting hands on with Server 2012 when I get off my summer projects.
Yeah, that doesn't look too bad. No offence, but I was thinking consumer NAS rather than business.
Is it 10Gb?
I meant 10Gb network connectivity. No, I've never personally seen a NAS being used - it's always SAN or local storage in my world. I know it's possible in theory, but I'm still not convinced by performance.
The SAN's I've been involved in normally use Fiber Channel or iSCSI - iSCSI is by far the most popular because it's quick enough for most things, but doesn't need any extra infrastructure. You'd definitely want iSCSI on its own VLAN, though.
It's definitely possible to roll your own, but I wouldn't know where to start. You'd almost certainly be needing to run Linux, I can't imagine a Windows based box having the performance even if it's possible.
If you've only got one ESXi server (And no vSphere), then I'm not sure that a small SAN or expensive NAS is going to give you a huge amount of benefit. If you have vSphere and a couple of servers then I'd say you definitely want it for the benefits of live migration and all that jazz.
I'm about to get some brand new servers running Hyper-V on them, with a couple of Equallogic 10Gb SANs. Should be pretty nippy.
I'll let you know what it's like when it's all in. I have only mucked about with ESX3 briefly though, so I'm probably not much good at comparing Hyper-V to VMWare.
You might want to have a look at XEN Server. They offer an absolutely free edition (based on agreements of their take-over of the open source software). The free edition does not provide enterprise features like dynamic memory allocation, but the licensed version does (add-on product).
I have tested several virtualization platforms, but my fav is XEN. It's performance is incredible. Give it a try!