You're right about the competing product lines and there are a lot of questions raised by it. However, I don't think it's Solaris/AIX, it's Solaris/Linux ... AIX is still alive and kicking, but inside IBM there is a lot of movement and development on Linux. AIX is receiving incremental updates for newer POWER machines, but nothing new. For example, there isn't an AIX port for the Cell architecture, but running Linux on Cell was a given from day one.
That said, I don't think there's a question of which OS would win in an IBM buyout of Sun... Linux trounces Solaris because IBM has put millions of dollars into Linux for truly first class support of POWER, getting the kernel to support all of the nice features like hotplug memory add / remove and other really neat hardware support. Solaris would have to be ported to POWER from scratch and, in turn, have millions spent on it just to get it up to par with Linux which doesn't make any sense. And even with cool features that Linux doesn't do (ahem, DTrace), it would take far less effort to just update and improve existing Linux based tools (SystemTap) or develop a new tool from scratch, rather than dumping money into the OS just to get them.