The F5 is not that horrible on batteries, especially if you use lithiums. It does use more battery power than the F100 does, but I haven't found the consumption to be excessive.
I have both cameras. I got the F100 new. I love the camera. The ability to take off the MB-15 is nice when size and weight is an issue.
The F5 handles just as nicely. All the controls are in the same place, which makes moving back and forth between the cameras really simple. The autofocus is slightly better, but not dramatically so. The motor drive is much faster, but that's not always significant.
One thing I like about the F100 over the F5 is the viewfinder. When you manually select a focus point on the F100, it turns red and is quite easy to see. On the F5, if you have the right focusing screen the point turns dark grey, which is fairly easy to see in the daylight but in darker conditions is not so easy. There are also arrows that point to the sensor from the side of the frame, but I don't find them to be all that intuitive.
I like my F5 better than my F100... but the F100 is not bad. What will your second Nikon be? (Every film Nikon shooter should have at least a pair of bodies, I think. Different films, different lenses...). If you are short on money, buying a pair of F100s might serve you better than having a single F5. If you already have a nice Nikon body, then the F5 might just be perfect.