So long as she's not super young, I don't see why she can't have devoted time to both pursuits. I mean, as long as she's not the type of person who never studies and yet has achieved super awesome mastery status as a summoner and a white mage, and is admired by all and sundry for her genius level abilities and has a bevy of followers who admire her and . . . you see where I'm going. In real life there are people who actively pursue their interests when they're young, and as a result can claim some marvelous accomplishments before they hit 30. It all depends on her natural ability and her determination and devotion to make up for the things she lacks (or to supplement what she already has). In fact, having her be rather successful with one and struggle with the other could give her something to agonize over, just as for most people they have subjects that they excel at and others that they have to struggle and work harder than others to see the same results.
There's nothing wrong with having a talented character. To me, the trap people tend to fall into is making the character be about the talent/feature/tragic past/whatever rather than consider the whole of the character. I think that's where the term Mary Sue can be awkward. It's entirely possible to take the supposed traits of a Mary Sue and still have an engaging, interesting, and sympathetic character come out the other side -- it's all in how you write it. (one example off the top of my head is Honor Harrington. I can definitely see several traits that could give people reason to call her a Mary Sue, yet that character is one of my favorite sci-fi heroines)
So I say do what you want and what makes sense for the character. If that means both, then go for it!
There's nothing wrong with having a talented character. To me, the trap people tend to fall into is making the character be about the talent/feature/tragic past/whatever rather than consider the whole of the character. I think that's where the term Mary Sue can be awkward. It's entirely possible to take the supposed traits of a Mary Sue and still have an engaging, interesting, and sympathetic character come out the other side -- it's all in how you write it. (one example off the top of my head is Honor Harrington. I can definitely see several traits that could give people reason to call her a Mary Sue, yet that character is one of my favorite sci-fi heroines)
So I say do what you want and what makes sense for the character. If that means both, then go for it!