Choosing a package manager is easy: Use either yum/apt and put smart on top of it.
yum means the underlying package format should be rpm, while apt means the underlying package format should be dpkg.
Both rpm and dpkg today are good enough in their own right.
package formats also usually imply that the build recipes should be based on existing recipes which produce output in those formats (e.g. Fedora/CentOS for rpm packages, Ubuntu/Debian for dpkg packages).
The Belenix repositories at present have recipes in the form of spec files - these are in a format different from the Debian world. The package build recipes too are borrowed from SFE/Fedora. At present, the package quality at Fedora/CentOS is definitely much higher than that at Ubuntu. Debian packages are definitely of high quality, but one concern is that the packages are not updated as regularly as we'd like them to be.
Another concern is whether the Debian community would welcome contributions (such as Nexenta's enhancements to apt to support useful Solaris notions). This is a dilemma that can be easily resolved by talking to Debian, I think, even though they have in the past lashed out at well-meaning posts by Nexenta folk. This blog post (http://ianmurdock.com/solaris/no-good-deed-goes-unpunished/) makes things a bit confusing, though.
Update: Removed the Music label. The opensolaris planet was picking up the music label as the title of the post !