Also keep in mind a proper 5.1 setup will used Storage Profiles that may have many datastores - you will not be able to define what datastore is used (unless you make lots of profiles and control that way).
For 1.5 I have not seen a guide but here is how datastore is picked (from my testing):
pVDC is selected (via OrgVDC)
All enabled datastores are put in a list (from the pVDC options)
If using FastProvision:
* List is reduced to datastores that are NOT in the "red" and contain a shadow copy currently. (so link can be done)
* If list has nothing then list is reverted to previous state
Of the datastores on the list the one that will have the most % free after the deployment is done is picked. (This is done by taking current used space on each and adding the MAX possible space usage of the vApp; not the thin size but full size - and seeing what would be left)