Adding storage to VM Best practice

I everyone I have a quick question, maybe someone can help me with this. Here's our environnement:

ESX 3.5 Update 4

Storage: SAN FC

We just added 2 Tb to our SAN and it created a 2nd vmfs volume. So now we have storage VMFS-1 and VMFS-2. Both 2 Tb each.

Here's my question: I created a new VM(FTP server) with a storage of 40Go (on VMFS-1). Now I want to add a new partition of 500Go to this VM. I have about 500Go left on the volume VMFS-1 so should I just add 500Go in size to the existing capacity in the VM settings? Or should I add a new hard drive to this vm(in edit settings) and create a new virtual disk and point it to my new volume (VMFS-2) that has noting on it yet? Is there a performance difference between the two methods?

Thanks for your help...