Obviously the smaller voltage range is less desirable - It's an artifact of using a different silicon technology to achieve additional power savings. If you can live with the 3V range, the LF parts can save you power. If you need 5V, or if you need a part with a better sense of humor about transients, or if minimum power is of lesser consideration, the F parts are the way to go. The actual logical structure, and hence behavior of the devices is identical in my experience, so the same code runs on either.