Adam Bertram explains the difference between the where-object and the where method in Powershell:
The Where-Object command is a sort of generic filtering command. Use this command to filter any kind of object in PowerShell. The Where-Object command has a FilterScript parameter, which is a scriptblock that allows the user to place code in it. If this scriptblock contains code that returns anything but $false, $null, or an empty string, it will allow whichever object the user passes to it.
For example, let’s say I’ve got a folder full of files. I’d like to see only text files and only those text files modified today. To make this happen, I can use the provider-specific filter with the Get-ChildItem command and also the Where-Object command.
Read on to see how that compares to the where method. Given the latter’s limitations, I’ll probably stick to where-object anytime performance is not critical.