The M1A2C and the XM1A3 imo can not be compared, they share the same weak spot under the gun that can make them an easy target both in PvE and in PvP at close/mid range, and the T10 bounces a lot more with the side, if is moving and the enemy from a long distance can not aim at its weak spots, but the difference is all in the gun.
While the T9 has an average gun the T10 has very high pen with the double shot one and a really high damage, specially if it uses HEAT, at the cost of some accuracy with the 140mm one. Even without Cortez, that I don't have, I get often 1200+ dmg on Bradley and some other tanks in PvE, and the high alpha makes very easy to set fires and is sometimes possible to ammo rack a T15 with a single shot, with 2 you do it almost always if you know what to aim. When I play the 2 tanks I really feel that there is way more then a single tier of difference, to me they feel like having 2 tier difference at worst.
As the T9 MBTs are anyway somehow sub par, as in PvE have to fight the same enemies you find with T10 and often they have to compete with them as you are in a T9-10 party and in PvP it is often the same, but worst as you have often to face T10 MBTs driven by humans, so you can not use the PvE tactics and trick to survive, only some times you get in a T8-9 Random Battle or GLOP, it is probably more important to focus on what a T9 tank can unlock then how it is good. Unless the tank is used to gain (player) experience in PvP, then the MTTB is surely better. But they both perform well in PvE, if the player is able to make them shine, other way they will be constantly out damaged by the T10 MBTs and other tanks like T15, AS21 or Wilk that have a much higher dpm.
And even if i like very much the GAU, that is a tank that in some PvE maps can do an incredible amount of damage and even spot in some situations, with the right retros and Sabrina or Erin as commander and using the bushes, and is viable also in PvP, even if needs much more finesse ( I got my single gold medal with it in RB mostly playing near the MBTs that was brawling and peeking out from a ridge exposing only part of my turret while i was melting the enemies occupied in fighting vs my party mates, not sniping from far) I think that the tanks that are unlocked by the T9 American are more interesting options, you have get a lot of xp to have the GAU really functional and tanks Sphinx and XM1A3 are very good tanks and Leo 2AX has the better gun of all the MBTs, very accurate, good damage and very strong HEAT, and as long as you learn how to use it has also decent protection, even if obviously is not a 490 or an ADTU, but at least his weak spot is the LFP that is way easier to protect then the spot under the gun.