Brno is a lot larger than Maribor, if I'm not mistaken, so more likely to have museums and other attractions. It's also nearer to Vienna and with a rather better train service (a through train every 2 hours, plus connections).
I've only had a brief visit to Maribor, but as I recall it has a pleasant historic city centre, with a range of big hills immediately to the north and west, so probably opportunities for more outdoor activities. The rail service is two through trains a day, plus sporadic connections with a change at Graz and Spielfeld Strass (the Austrian border station).