I too agree about the 50mm objectives at the higher magnification settings. But you really have to ask yourself two questions:
1) will you really be using power over 12x? and
2) will you really be shooting during low-light conditions?
If the answer is yes to both, then I recommend the 50mm.
Also keep in mind that, even if you will use power under 12x and will shoot during bright conditions, then bigger objectives are still helpful because, since the light being transmitted is "thicker" - meaning 4mm or 4.5mm or 7mm, or so on, that means that you are have more "working room" to adjust your eye. Otherwise, when you use high powers and smaller objectives, you have a narrow window within which to look - so if you move your eye a little too high or low, or left or right, you'll loose sight. As such, a larger objective is more forgiving to your eye alignment.
Zachary