It makes sense to use the ServerManager class in this context if you have multiple user identities hitting on your webpage and you want to use those identities to connect to the PI Server.
The recommended way is to have a shared ServerManager for all pages to see in order to optimize connections (just like WebParts do).
If you only have one identity hitting on the page then using the PI SDK object would work fine.
Note that in either case having a ServerManager will work and you could scalate later without changing your code, but if you use PI SDK and later on you decide to go for a multi identity approach then you would have to go back to the code to change things.
Also remember that ServerManager will not work with explicit login to the PI Server.