I like this better if
Ballistae are buyable items (because they otherwise aren't common enough and aren't placed conveniently enough)
The 6 move goes to the Hunter instead (so you have a class that tries to get off the really long shots, and is accurate given time to aim, and thus doesn't need the movement; and then the class that runs around going LOLARROWZ).
Edit: I don't think the "trench warfare" issue is a big problem, depending on how many of these units there are. Storyline can justify stuff, too. Maybe ballistae are expensive and the enemy can't afford to arm a whole bunch of them. Maybe there are story-justified turn limits, and the blue team isn't going to have a lot of either type of unit anyway. Maybe a good idea to disable crit on ballistae, though.