A couple of weeks ago I got an email from somebody with Wintel certification and some ambition asking whether it would be better to study AIX or Solaris. I sent him an instant response - and want to elaborate on it here.
I think the answer is that "it depends"...
I generally favor Solaris and the BSDs over Linux, Linux over HP-UX, and HP-UX over AIX; so if the question comes down to which is the better implementation of the core Unix ideas, my answer is unambiguous: it's Solaris, and by a decadal margin.
If, on the other, the question is which Unix offers the best employment opportunities, then that's a whole different game and the answer depends on what you want out of your professional life.
In my experience most employers using Solaris fall very broadly into one of three categories:
Any decent certification will get you an interview with the data processors, but Solaris expertise won't get you the job - on the contrary, they'll treat you with suspicion and any betrayal of core Solaris attitudes (like a focus on getting the job, not the paperwork, done quickly and efficiently) will cost you the opportunity.
The legacy continuation people can offer decent places to work, but are often deeply conflicted - with the Unix people a tiny and deprecated minority among hordes of Wintel pushers.
In the very worst cases getting a job at companies like this means you'll be responsible for the major applications supporting the business; but IT management will deny you user contact; you'll get treated like a leper; and, when a real change becomes necessary your bosses will fight long and hard against upgrading your systems - often proving to user management how expensive and unreliable Unix is by hiring experts from their favorite wintel consultancy to give you a hand in coping with the horror of it all.
In contrast, the people who run Solaris simply because it works for whatever they want to do are usually great to work with, but won't look at you until you can prove both experience and commitment - generally by being willing and able to take on roles far beyond systems administration.
The AIX world is significantly less fragmented with most users simply deep blue loyalists and only a relatively recent handful adopting it because they don't like Sun and there really is no other practical choice where security concerns rule out x86.
As a result employers looking for AIX expertise are generally (not always, of course, but generally) likely to be still following 1920s job role and control standards in the data center. As a result certifications count far more than do in the general Unix world, and your job will be far more tightly circumscribed - i.e. the job you do will be the job you're hired to, and absolutely nothing beyond that will be considered in scope for you.
Interestingly AIX itself echoes this difference in a very fundamental way: it's very good at breaking system resources into separately managed pieces, but not good at combining free resources to meet emerging needs - where Solaris, properly used, is the opposite: always capable of providing true SMP across the machine because containerization is really just virtual virtualization:
Both OSes work for most jobs most of the time - but Solaris is moving in the Plan9 one OS, many boxes, direction where AIX is still really just the core late 80s, early 90s, single box Unix it started out to be -although now with extensions to accommodate modern hardware and lots of incongruous stuff pasted in to meet the data processing community's role and control expectations.
The consequences of this for your job are pretty direct: with AIX you'll spend your time working with AIX - but because Solaris is largely a set and forget environment you'll spend most of your time there working with applications, not the OS.
Boil it all down and the bottom line for you is simple: if you want to be a deep dipped super techie who gets things done for users - then go with Solaris on SPARC. If, on the other hand, you want an insulated 9-5 job with no responsibilities beyond conformance - then pick AIX.
But remember this choice is artificial: there's a happy middle way you'll want to consider: learn to use Linux well first, and then go after Solaris and AIX certifications on an opportunistic basis.