What is the difference between current T2 balance and SupCom experimentals?

Okay. What would you describe as a "sidegrade"? Because it sounds to me like "the same, except different".

To some people everything is the same because they boil it down too much.

A unit that is a sidegrade would be similar to it's other sidegrades while having specific strengths AND weaknesses that their other sidegrades do not have.
For example (assuming all things like cost and health the same):
Ant could have a DPS of X
Leveler could have a DPS of 1.5xX but a fire rate of half. This way, it is at a serious disadvantage against units while being exceptionally good at taking down buildings.
So yes, similar but different.

I guess the biggest thing right now is that they both have very similar roles and yes, before long the T2 tank just replaces the T1 tank.

However, I would still say the way other units interact with these two tanks are fairly different. Catapults love picking off a T2 tank force compared to picking off T1 tanks. T1 bombers slaughter T1 tank swarms, but are less effective vs T2 tanks. This pretty much boils down to how much overkill is involved. Units with lots of overkill will perform slightly worse vs T1 tanks compared to T2 tanks. If your opponent doesn't have a lot of aoe, but primarily high damaging single target weapons, I would send T1 tanks after them.