Antivenom was my first Uni and he does great. No only is he essential in WBU, but he comes with full attack speed, full movement speed, some ignore dodge, and full HP recovery in battle.
He can clear anything for me...aside from later WBU stages and Odin.
If its between Anti-venom and x23 (assuming you've gotten all the meta unis such as IW WS, Cable, etc), definitely go for antivenom. He's one of the best WBU strikers as he strikes on attack and drops those coveted heal blobs; in addition he clear pretty much any shadowland stage he qualifies for, bar perhaps 27 and 29 without an OP build.