I'd always get a mouse with a charging dock.
My last Logitech mouse lasted about a month/6 weeks on a set of batteries - and it annoyed me.
I now have a Logitech Laser mouse, with a battery meter - once it gets to red, I just dock it for a few hours and it's back at full charge.
Batteries in keyboard last months tho!
You can get Logitech sets now with the Laser mouse as part of the bundle - well worth it IMO.