Medium vs heavy tanks as axis

a heavy tank is approximately twice as strong in both xp and damage, but only 50 percent more expensive?

Though it could be said it's more expensive when putting in mind the Extra research through lvl1 mediums and upgrading to level 3 tank plants, I would day it's a one time investment. After that it's much stronger tanks for proportionally less costs.

As for speed,

As an artillery-focused army, all significant battles with major stacks will need my very slow rrg and artillery anyways, so does speed matter significantly? I do have motorized infantry and/or ACs for fast land grabbing but neither are used for proper combat anyways, just scouts and land grabbers.

Thus I would say that keeping my heavy tanks with my infantry will make a fantastic mele stack in front of my artillery if push comes to shove.

Then why not invest in heavy tanks if we are investing in medium tanks anyway?