My choice is none of the above:
However, of the options, I like Baird if you want an aggro card and Kayla if you want a more midrange-to-slow card.
In order from most to least desirable of the others, imo:
Getaway is a
Wildfire, if you're doing that in the occasionals already that's a strong point in its favor.
Velomachus is great fun when it arrives but it costs seven and frankly half the time you're hitting garbage (not like Arena in constructed when you had so many
Time Warps that they had to banhammer).
Purification is a
Cataclysm, same logic.
Spark Trooper's whatever. Never gonna get me to go into RW or even want to splash it, honestly, if I don't already have fixing. And that's not good enough to justify the slot for me.
Trenches is sooooo sloooooow and I hate it so much. It makes ground combat a miserable slog and it doesn't even kill fast enough to end the game and put me out of my misery.