My comments are based on what I see in my own area. We don't run stacker planters (or any 3ph planter) in this area, so I don't know what works best with those. More typical for us is something like the 1570 cart with a 1520 drill that I linked to (Great Plains setups are also common). 15' wide behind 100 to 150 hp. Lots of smaller field sizes with tighter corners. Typically you might have a 300 gallon starter tank on the cart. In this situation, yes, I'd think the cart mounted tank would be more agile. We're also not likely to have a tank that can be towed, so that would have to be purchased.

I can easily envision other situations that would be opposite. I think your scenario might be one of them. I take it you won't need the coulters in front of the planter (starter going straight into the row?), and are capable of carrying the planter. So adding the cart strictly to carry a tank (small at that) does seem like an expensive way to move some liquid around the field. In our area, we go to trail planters with frame mounted tanks to do this.