Sharepoint - When to use scriptlink, scriptblock and script

It really could depend on when you are looking to load your js file. With ScriptLink tag you have the ability to specify when you want the script to load asynchronously or after the page is ready. The ScriptLink will always look in /_LAYOUTS/1033 folder for the scripts.

<ScriptLink language="javascript" name="MyJS.js" Defer="true" runat="server"/>

This line would load the page and then load the script file. If you set Defer="false" then instantly the page loaded and hit that line the .js file would execute.

Using Defer="False" is basically the same as loading the script with this method. But with this method below you can specify a location other that LAYOUTS/1033 if you wanted to

<script type='text/javascript' src="/_layouts/1033/myjs.js"></script>

Another difference with using the SharePoint:ScriptLink is that when the file gets loaded it must exist, otherwise you will get a SharePoint Error screen. This could help you ensure that SharePoint is seeing your js file

Besides referencing JavaScript files, your ASP.NET pages can have inline JavaScript code. Use the following pattern to make your script blocks

<SharePoint:ScriptBlock runat="server" >
    // Your JavaScript code here.
</SharePoint:ScriptBlock>

If you set the "Localizable" property of the ScriptLink to false you can also reference files outside of /_LAYOUTS/1033/ and just put your path to the "Name" property

    <SharePoint:ScriptLink Language="javascript" Localizable="false" Name="/_layouts/JS/myFile.js" runat="server"></SharePoint:ScriptLink>