Honestly, WTF?
Do you really need to have someone else's guidance on this?
If you want to have access to the songs when you don't have a network connection, put the songs on the local storage. If you don't, don't.
If you're not sure, try it both ways for a couple of days and see which way suits you.
Not trying to be a big prick (just a little one) but honestly, how are we to know what will work for you, with your usage pattern, your cell reception, etc.?
-Suntan