I like the way Dragon Quest handles this, though the exact mechanics wouldn't translate to D&D. In more generic terms, it boils down to this choice:
- Staying at home - slow XP gain, money being spent like water, learn whatever you can find a teacher for, with practically no risk.
- Adventuring - fast XP gain, money coming in pretty decent (as well as the DM provides and the players take advantage of), minimal need for teachers, lots of risk.
In DQ, it's not quite that cut and dried, because characters don't actually improve during adventuring. It's merely that if they have a ton of XP and money gained during an adventure, they can use time very efficiently to gain abilities as long as the XP and money holds out (and they are improving things they used on the adventure).
It's that last bit that ties in most clearly to the OP. Because DQ skills can be risky to learn, you can most take advantage of staying at home when learning something new--especially if you happened to succeed in an adventure that paid better in silver than XP.

So the mechanics have a mild encouragement to use adventuring to improve what you know, but staying at home to spread out.
To make something similar work out in Next along the lines of the OP, you'd like for your either/or abilities to be extensions or variations on the main abilities. For example, consider
fireball. The adventuring guy might get the option to enlarge the blast or change the range--extend the power of the spell. The stay at home guy might get the option to change the shape.
That might be too fine-grained, but it does suggest the possibility of unlocking such abilities over time, with each successive unlocking widening the differences, while still allowing a balanced character to do some of each. Alternately, you can also cap it by level--i.e. every level, you get to do one or two of these unlockings, with the mix allowed being somewhat of a DM judgment. If you home for a couple of years (off screen), then you can only pick those options.