First off i wanna say Happy Belated Birthday to you. One thing you could do is buy a pre-con commander deck and beef it up. One suggestion you could try is getting the
Kathril, Aspect warper and putting a
Skullbriar the Walking grave to the deck. It's one color shy of Atraxa and you can still enjoy the Proliferate mechanic if you wish to add it to the deck. If the choice comes down to the cards you suggested then i'd suggest the
Umezawa's Jitte for this deck as a possible build direction. Discard creatures using
Key to the city, Drop
Kathril aspect warper, give
skullbriar the walking grave numerous counters, make him unblockable for the turn and get some major swings against your opponents. IT's a strong card for a strong deck and the power creep won't be that bad because you could still play it in a tame manner (while steam-rolling your friends