Is it good for performance to sleep a Mac rather than shut it down? OSX Mountain Lion, Mid 2012 13" MacBook Pro.

I'd like to know what is the best practices about closing the Macbook Pro lid (sleeping) verses shutting it down?

I often simply close it during the night, leading to long uptimes (4 days right now).

Sometimes I feel it's not as snappy as it is when I rebooted it a few minutes ago.

Should I shut it down when I don't use it for 3+ hours for example? Will it help it lasting longer or am I being paranoid?
A long uptime is four days? My MBP has uptimes of weeks. Not as snappy? I guess you're running out of memory or something else is wrong. 

I've never had this issue where the machine is not as responsive after closing the lid as opposed to rebooting. I generally only reboot because of WiFi connection problems or software updates.

To answer your question - just close the lid. Only reboot when necessary.
When closing the lid it goes into a sleep state which I think is very nice because when you open the lid it is almost instantly awake and ready to be used. While if you power it off it will take some moments to be booted.

You could leave the lid open as well if you like. The fist thing that is probably is gonna fail is the battery. To keep that in good condition make sure you keep your battery charged (there are other topics about this how to do this and what is the best).

In the end, the machine is built to be used and you should just enjoy using it without thinking to much about what is best. Apple already has that done for you :).