MemoryReserveMB vs AvailableMemory in powershell to determine available memory for vms

Hello All,

I am trying to figure out available memory on host for virtual machines. 

when running Get-VMHost -ComputerName "hostname" it returns 2 related parameters:

AvailableMemory,

MemoryReserveMB

So before starting vm by script I would like to check is there enough space on it. What is correct way?

1. VmSize <  AvailableMemory

or

2.  VmSize  <  AvailableMemory -  MemoryReserveMB