Is it just a historical choice that both magnetic field and the Lorentz force equation include the speed of light? I figure that whoever wrote up the equations (in cgs!) could have put both factors of $c$ in either the force equation, or have define the magnetic field as being smaller by a factor of c- but they didn't. Any ideas why?
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There were historically several systems of units (ancestors to modern SI, CGS electric, CGS magnetic, CGS Gaussian, CGS by Heaviside), and the ultimate choice in favour of Gaussian CGS was made when Special Relativity has united electric and magnetic fields into one electromagnetic field tensor. Only in Gaussian (and Heavisidian) versions, these fields take no additional factors and make components of the field tensor immediately. Any other choice just looks ugly.
For the reference, the electromagnetic field tensor in CGS has a form $$F_{\mu\nu}=\left(\begin{array}{cccc}\hphantom{-}0&\hphantom{-}E_x&\hphantom{-}E_y&\hphantom{-}E_z\\-E_x&\hphantom{-}0&-B_z&\hphantom{-}B_y\\-E_y&\hphantom{-}B_z&\hphantom{-}0&-B_x\\-E_z&-B_y&\hphantom{-}B_x&\hphantom{-}0\end{array}\right)$$
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