By science, Getting another 2x10 is the thing to do (please Alex or Bill correct me if i get this bit wrong!)
-Stacking them end on end will bring the speakers much closer to ear level, Increasing percieved volume, as well as the dispersion around the stage being much more even.
And on a second note, If you bought a 410 or 115, Each individual speaker would be working under a different stress, for example (again, correct me if i get this wrong!) 300watts into two cabs, one 410 and one 210, They both recieve 150 watts of power, each cone in the 410 recieves 37.5 watts, where as each cone in the 210 will recieve 75 watts. Basically you wont get the most out of the 410 before you blow the speakers in the 210. Especially as your only using 300 watts.
Hope that made sense!