Sure, the IIf might be a more uncommon camera, but I highly doubt it's worth enough to buy a pair of IIIf's. Seriously, look at the online retailers of vintage Leica equipment (like ritzcam.com) and see for yourself. Ritzcam.com has a large selection of IIf and IIIf camera for sale right now, and the IIIf's are generally more expensive than the IIf's. Plus, the situation has been the same on auction sites like ebay. In this instance, I believe the demand for the IIIf is far greater than the demand for the IIf.
Rarity is not the only determinant for value. It's all about demand! If rarity was the only concern, then M2 cameras should be selling for about 3 times more than M3 Leicas (since there were only about 80,000 M2's made, versus about 230,000 M3's). And the Perfex Speed Candid would only be owned by the rich!
Back on topic, the IIf is still a useful camera, but as has been previously mentioned, lacks the luxury of a slow speed escapement. If you intend to use the camera with shutter speeds below 1/25th, I would pass on a II series Leica. However, remember you do have flash capability, which somewhat makes up for the lack of slow speeds. If all you are wanting is the option for 1sec., 1/2sec., etc. on the shutter dials, and flash use isn't important, you might look into a IIIc or IIIa. These cameras almost always sell for less than a similar condition IIIf. And this isn't just speculation based on rarity, it is a fact!