Antiprotons have an inherent +20 crit severity mod, while Tetryons get a 2.5% shield stripping proc. As a result, AP weapons will always enjoy a +20 crit-severity advantage over Tetryon weapons with identical mods / suffixes. When the the target has no shields, tetryon really becomes lack-luster compared to other energy weapon types.
That's why you would have to get Tetryon's that have an extra crit modifier to get the +20 crit-sev. Then they would be equivalent, as far as that goes.