Best Practise on Datastore Configuration

Which is the best way, and why?

1. A seperate target > datastore for each disk required
2. A single large target with multiple VM's residing inside it.

Basically my SAN has 2 arrays - standard 7.2k SATA drives and some ast 15k SAS drives.

For things like fileshare and OS, I am using the 7.2k array. For things like SQL Databases I am using the 15k drives.

So my current config shows like this:

SQL VM
Disk 1 - OS
Disk 2 - SQL Databases

Disk 1 is a target on the 7.2k drives, presented to VMWare as a single Datastore.
Disk 2 is a target on the 15k drives, presented to VMWare as another Datastore.

This makes sense to me.

Then:

Domain Controller
Disk 1 - OS
Disk 2 - FileShare

These are both on the 7.2k drives, but are presented as seperate Datastores.

Am I complicating things by keeping all of these disks seperate? Should I in fact create 1 single target for all 7.2k disks, and 1 single target for all 15k disks?