Actually, neither really makes you come to them.
The solar's ranged attack is rather unimpressive, except for the slaying arrow. 10 hp/round isn't going to win any kind of a ranged exchange. What the solar's ranged attack does do is it offers the opportunity to route key units (like a timber wolf) early in the game and get a points advantage while simultaneously being hard to harvest points from. (There's only 20 points you can get without killing either the solar, shield guardian, or bodyguard). The real threat of the solar band is the solar's melee ability when combined with resistances and the ability to spread damage around and consequently keep up the damage output when other bands slow down upon the loss of one or two hitters. The solar is also hardly immobile.
Arcane ballista bands, on the other hand, generally try to kill you before you get to them. If you can actually make it to the ballista and engage, you will usually win--the ballista excels if if can kill enough of your forces that you can't eliminate it once you get there or the ballista's player still has a points lead once you've eliminated the construct.
I think the Solar and Ballista are pretty good bands.. and they do exactly what immobile bands are susposed to do - make you come to them.