Yes, this applies to computers as well, restoring the system removes the caches, bugs, and other junk that you no longer use or has gone rotten.
I doubt this will do much help to you, but all rechargeable batteries need to be whats the word...i forgot but pretty much you should let the batter get to almost dead then completely recharging it. we do that once a month on our laptops at my school to have the batteries working well. having them plugged in all the time is not good for a battery. I mean doubtful especially for a phone.
the iphone will use more power then the itouch, and it does depend on your usage so it may be your fault.
The only other thing I may think of is weather conditions. The hotter it gets the more electrical devices need to cool themselves, this of course consumes more energy and since the summer just began, it may be another cause.