I agree with this. SmartOS would be my choice over Solaris. I think FreeBSD is getting there. With things like Tredly. The building blocks are solid and available they just lack some polish.
So if you have the skill FreeBSD. Otherwise SmartOS makes a killer setup for containers unmatched by anyone else.
A warning, though: don't try to use FreeBSD with SCSI tape drives, in my truncated experience because of what I found experience a few months ago, the driver is capable of writing without error data it then can't read back (but can be read back by Linux).
Maybe not so important in the container context, but sooner or later, somewhere you need persistence, and tape offers certain persistence features you pretty much can't get elsewhere, especially at its media price points.
(Plus, if they got this wrong, when it's really not that hard to get right (I've done SCSI at this level before), I wonder what else they have.)
So if you have the skill FreeBSD. Otherwise SmartOS makes a killer setup for containers unmatched by anyone else.