Personally, I think your crazy for worrying about it. How will you get uneven deflection? The pushrod can't move side-to-side because the control horns are holding it in place.

I used the "Y" pushrod arrangement many times without any problem whatsoever, including on my Super Skybolt which is easily as aerobatic as a Citabria (If not more).

If you want to get into the geometry of it, then yes, there may be a MINUTE bit of difference between the movement of the two halves, but it will be so small that you'd probably get the same amount or more from using two servos (unless they are extremely expensive servos).

I say go with the planned pushrod and don't loose any sleep over it.