The last figures I saw shows IBM grossing $5B per year from
AIX and a somewhat trivial amount from Linux. That was 2004 time frame I think.
I'm very biased towards
AIX but I just don't see Linux as a competitor at all. Linux may be 10,000 times better than MS's stuff. But
AIX is 10,000 times that again. My office machine was often up for 18 months at a time and was rebooted only because they tested the buildings emergency power circuitry.
But I'm also very skeptical of IBM's efforts. When the RS/6000 was first released, there was a "porting center" in Austin. There were programs to get advanced information to developers. And release 3 was very well documented for an internal developers perspective. The release 3 edict was to document everything. Don't forget, this was during the "Open Software Fondation" time frame which IBM was a part of.
But by the time release 5 got here, all those things were gone. There are many items that are not documented in
AIX. The port center was long gone. Third party driver and kernel development had vanished. I was on a task force to try and reinvent a development kit but nothing came from it.
Things that hard core developers like me would like to know are just not in the pubs. For me, that is a slight advantage. I was inside up until December of 2005. So I know tons of things that are not documented anywhere. But it would still be very nice to have them documented.