True, the IT staff makes the difference, but the management makes the IT staff. Should you be in a hapless IT staff position with a Fortune 1000 company that just focuses on Wall Street metrics, my apologies (I have been there). You can count on the staffing problems to only get worse year over year. Seriously, you should consider changing jobs -- and that is where the particular DBMS comes into play.
I started out as a mainframe DBA going back to the 1980s (hierarchical databases). I went on to mainframe DB2, Unix, and SQL Server. The best change I made was leaving Big Iron. Why? Not because of the technology. DB2 is by far has the greatest ability to scale, has true continuous availability (I don't play games with the definition of "24x7"), and simplicity. The reason I left it was it is a victim of its own success.
Big companies use mainframes (the market is actually growing in the Far East and south Asia) and big companies are getting more and more oppressive. More and more (including our favorite software vendor) have adopted the "rank and yank" approach to the annual performance review. Ranking has been around forever as a management requisite, but it is the yanking that is being now being crudely done. In the name of quality, management has to rate a certain percentage of the staff as under-performers. After a couple of years of this, you are let go (to meet Wall Street financial targets). The problem is it works just the opposite -- it generates mediocrity.
If you are locked into the mainframe, you will probably be locked into the "rank and yank" for the rest of your career because you will only change to another large company with the same mentality. If you are locked into Unix, you still might find this happening, but less so. If you are in SQL Server, there are plenty of small to medium companies to go to.
Am I cynical? Not really. I am really trying to give you some options. Smaller companies are more personality based (rather than process-model based), so you will have to find the company that matches your personality. Once you do, I think you will make the most out of you database career. Afterwards, you will probably agree with me that the platform really doesn't make that much difference.