I want to install two VMWare ESX with HA option. Can I store virtual machines in a SAN and share the LUN betwen this two ESX hosts?. What is the advantages of Vmotion for hight availability pourpose?
You dont' want HA with just 2 servers. HA uses the equivalent of a machine (default configuration, can be higher) to bring up VM's in case a host goes down.
In this case you only have 2 machines, which means you would have only 1 if you use HA, bad idea. You need at least 4 ESX servers or to make HA work well other wise you are wasting resource.
vmotion allows you move machines between hosts so you can performe maintenace or balance VM performance.
"You dont' want HA with just 2 servers. ... You need at least 4 ESX servers or to make HA work well other wise you are wasting resource."
To paraphrase the rest of your post: Regardless of how many servers you need to service your capacity requirements, the number of additional hosts you need to service HA remains pretty much constant (assuming N+1 redundancy throughout.)
I'm not too sure that the business case for a single extra host for HA is any harder to justify if you have one host than if you have eight. I'd much rather have the warm fuzzy feeling that my servers have redundancy than the knowledge that I saved a couple of bucks because it didn't seem worth it.