The question is which firm is worth more. Both have their values, especially in the patent areas. In terms of just smartphones, Microsoft would probably gain more from RIM, because it could integrate BlackBerry Enterprise Server into its own server products. Nokia, though, is a much older player and probably has a lot more of a patent portfolio.
The question then becomes which is an easier purchase. Nokia is a 150-year-old storied company. The Finns may not be too keen to let it go to an American firm.
There is the distinct possibility Microsoft acquires both firms and keeps the best of both worlds for hardware. But where does that leave OEM partners like LG, HTC and ZTE?