AIX doesn't begin to compare to Solaris in binary compatibility or investment protection
"If an application runs on Solaris 2.6, 7, 8 or 9, it will run on Solaris 10, even if it has not been recompiled for Solaris 10—guaranteed."
IBM has NEVER had the bollocks to guarantee AIX apps would work from one release to the next. And they broke binary compatibility for 64-bit apps from AIX 4.3.3 to AIX 5.1. More technically, they broke binary from 4.3.3 to AIX 5.0, but AIX 5.0 was a complete failure of a product.
I'm glad IBM started to finally listen to UNIX customers. AIX's lifecycle management was simply horrible up until the last few years. Versions would change every year, and the life was only a few years for each version:
AIX 4.1, July 1994
AIX 4.2, April 1996
AIX 4.3, October 1997
AIX 4.3.1, April 1998
AIX 4.3.2, October 1998
AIX 4.3.3, September 1999
AIX 5.0, October 2000
AIX 5.1, April 2001
AIX 5.2, October 2002
AIX 5.3, August 2004
IBM used to release a new version of AIX every six months until 1999. Then it still released new version of AIX every year. With each new release, IBM would EOL the n-2 release. IBM AIX version 4.3.3 and 5.1 had lifecycles of about 3 years. The rapid transitions between AIX 4.3.3, AIX 5.1, and AIX 5.2 were just too much for enterprise customers. Add to that IBM's constant mixed messages around Linux on POWER being the future, and AIX being little more than a stopgap. ISV certifications on AIX 5.2 plummeted because the ISVs got so confused about what the real OS of the future was for pSeries. It was a complete replay of the OS/2 vs. Windows debates of 1990. In 2002, IBM significantly slowed the AIX train, and backed off of the Linux on POWER messaging.
Note Sun got of of the semiannual OS crazy train in 1994, and went to a greater than one-year cycle after Solaris 7 in 1998:
Solaris 2.4, November 1994
Solaris 2.5, November 1995
Solaris 2.5.1, May 1996
Solaris 2.6, July 1997
Solaris 7, November 1998
Solaris 8, February 2000
Solaris 9, May 2002
Solaris 10, January 2005
Sun also has such a long support cycle on its Solaris releases, it is not unusual for customers to take the approach of adopting every other release, i.e., Solaris 2.6, Solaris 8, and now Solaris 10. While a new hardware platform may drive the move to a new Solaris release, typically this occurs a year or more after the release of the new Solaris version, and typically after ISV apps support crosses 1,500 certified applications.
"Now, try to install Solaris 8 on SPARC IV+"
Huh? That is a strange point to bring up in May of 2007, a year and a half after Sun released UltraSPARC IV+. Solaris 8 was released in February 2000, just 5 months after AIX 4.3.3. US-IV+ was released in September of 2005. Your example is the equivalent of installing AIX 4.3.3 (1999) on POWER5 (2004). I don't think you can do that. Solaris 9, released in May of 2002, over three years prior to the release of US-IV+, is supported on US-IV+. Three years is the same amount of time between the release of AIX 5.3 and the release of POWER6.
And if your point is about AIX 5.3 support on POWER6, realize AIX 5.3 is just 5 months older than Solaris 10! And given the much greater ISV support of Solaris over AIX, Solaris 10 is more like AIX 5.1 in number of supported ISV applications.
So with about as many ISV certified applications as AIX 5.1, 5.2, and 5.3 combined, SPARC customers are being well-served by Solaris 10.
Unfortunately, IBM's confusion around operating systems seemed to distract IBM from any real innovations in AIX. Most new versions of AIX simply added new hardware support, or better SVR4 compatibility. Meanwhile Sun really shocked the world with Solaris 10. Features like Dtrace, Zones, SMF, ZFS, etc. fundamentally showed operating systems do matter, and innovation in operating systems matters. Yes, AIX 5.4 Virtual Memory Protection Domains look very cool. But they are POWER6-only. Solaris tools like Dtrace (AIX has nothing like it) not only work on older, current, and future SPARC platforms, but also on AMD Opteron and Intel Xeon platforms.