In my modest experience with both mgs I believe that they are about the same level of difficulty. Both types have become rather scarce and spendy these days. Parts do not interchange by the way. The Vickers employs an inverted toggle lock.
There are various models of Maxims that have non-interchangeable parts. The WW-I German Maxim was the 1908 (and 08/15) model and the Russian Maxim was the 1910 model as was the Swiss Maxim. The main difference among these was the method of lock attachment to the toggle mechanism. The Swiss lock can interchange onto the Russian Maxim if you want a rimless cartridge conversion.