Gallardo by a mile.
Gallardo is a month or two away from the majors at most. Maybin is probably at least two years away.
Gallardo is a #1 starter for an up-and coming team who should dominate a weak division for the next 5 or so years.
Maybin is an extremely fast OF, who Ks way too much and doesn't show any significant power yet. His GB/FB ratio is extremely unnatractive and when coupled with his equally poor LD%, doesn't support his current batting average.
The apologists will talk about how Maybin has yet to fill-in, or has been injured, or is young, or blah blah blah. Why risk it? Gallardo is only 2 years older, but is dominating two levels higher.