How long the guitar stays in tune will depend on many things...how much you play, what gauge the strings are, how much the tremlo bar is used, etc. However, when you are selecting a guitar this shouldn't be a major issue, unless the brand or model you are buying is notorious for bad craftsmenship that causes it to detune or something to that extent. (Which you won't find as much in bigger name brands)
I would recommend trying out whichever guitars you are considering, and then choosing the one that you are most comfortable with..or the one that you has a good tone for you.
Ryan Buckner, Ysrafel