Legacy might be the better choice for playing against the CPU, especially on the higher difficulty levels, but to call it the more realistic system is highly debatable; both systems have issues. Legacy was replaced because of how unrealistic (and easy) it was/is, and the homing missile OP sprint tackles are still there (players don't sprint and make a standing tackle in real life like they can with Legacy defending).
If we're being honest, both systems kinda suck in single player. In multiplayer, Tactical actually works great and is far more realistic. That said, I might switch to Legacy because while tactical defending works fine with the right slider adjustments, I prefer closer to default settings and Tactical defending just isn't a fair system against the CPU, especially on WC/Legendary.