Also, if you were thinking about depth-of-field, consider that the extra focal length more than compensates for the loss of stop.
For example at 3m focus distance.
70mm @ f2.8 => 31cm DOF.
105mm @ f4.0 => 19cm DOF.
Except... you have to shoot from further away to get the same subject framing with the longer lens, cancelling out the differences in DOF. With the same framing (shooting from further away with the 105mm lens), the shorter lens would actually have less DOF at f/2.8 compared to the longer lens at f/4. ;-)
At most distances you'd be concerned about D0F, you'll have the same DOF regardless of focal length, if the aperture and subject framing is the same.
Here is an article on the subject, and you can check it with a DOF calculator (works, unless your focus distance gets close to the hyperfocal distance).
IOW, if you shoot with a 100mm lens, you'd need to be twice as far from your subject for the same framing as you'd need to be shooting with a 50mm lens. So, you have to take framing into consideration calculating DOF.
Although, I'll admit that the perspective difference you get from shooting from further away with a longer lens gives the illusion of a shallower depth of field (due to the more compressed background).