I wanted two cards for grindy matchups/mirror. The options are basically Bloodbaron/Baneslayer, BAtterskull, 2nd Gideon (or 4th Rhino), Painful Truths or a planeswalker. Ideally the card would be good against BGx, Grixis Variants, Twin (post board they ALWAYS grind) and Collected Company Zoo decks. The lifegain options are better against zoo, but not resilient enough vs BG, Grixis. So it came down to a planeswalker or second painful truths. Truths isn't very good vs Zoo and because of the reduced lifegain in my version drawing 2 can be a stress on your life total even in the control matchups (where I am keeping in thoughtseize and want to fetch shocks). The planeswalker I tried were Gideon, Sorin, Little Elspeth, Gideon New and Big Elspeth. Vs Path matchups (Junk, UWR, Zoo) big elspeth was just the best. Shes obviously the most powerful and you get pathed. Realistically Little Gideon/Elspeth might be better (sorin to fragile vs Bolt/Kcommand), but Big Elspeth is my pet card (won me a GP).
Gideon Jura over 4th Rhino. Worse vs Burn. Better vs Affinity/Infect, *****ty creature decks (like GW HAtebears). Neutral vs most other matchups. Gideon Overperformed by a lot. Note Brian Liu (who tested with me) was so impressed he played 2 in the GP in his zoo deck and got 16th. Gideon is a powerful, resilient anti small creature effect. in particular the matchups that hes good in are very close game 1, so I like having a very powerful card.
Shambling Vents. The most important part, is that it just makes your mana way better than something like treetop village. I build my manabases by focusing on color requirements first, because Junk wins games when it can deploy its cards. However, incidental lifegain was also important in my build (you could play + 1 stirring, +1 swamp, -1 forest , -1 shambling and have same mana). I had already shaved a rhino and a ooze. But your life total is under pressure from your own cards (mana base and painful truths/thoughtseize). I think generally speaking Stirring Wildwood/treetio is a better card in matchups where manlands are at their best. But those matchups are already all decent. Whereas in the other matchups Vents is better (mostly because it just makes your mana better).