The ATR is much more effective against armor than just the extra penetration:the actual TK DR required is typically 1 more pip than an LMG shot - making it a bit more likely to get an effect with the ATR. That said, the LMG has the possibility of an ROF 2nd shot that the ATR never gets.
* A Final TK DR = the Final TK# automatically Immobilizes or Shocks, unlike an MG where it will only Stun
* A Final TK DR 1 > the Final TK# is a possible Shock, unlike an MG which gets nothing
* An ATR can Deliberately Immobilize an enemy AFV (a change to the ASLRB that I vehemently disagreed with), which an MG cannot do
* An ATR can get a CH, which an MG cannot achieve, making it possible to burn a King Tiger with an ATR, IIRC
The MG is, of course, much more effective against infantry, and better all around, but the AT effectiveness clearly goes to the ATR by a mile.