Palencia is a clear call. Like you mention he is a pretty similar profile to Pearson, and on top of that he's absolutely shoving to start the year (and frankly, going back to middle of last year).
The other guy is where there's arguments in a couple directions:
- Ideally to your point it'd be a lefty. That said Cosgrove hasn't pitched a game in the org yet, Little's looking rusty, and Martin hasn't been sharp yet. If you feel dead set on a lefty I think Brandon Hughes is throwing the best *right now* though he requires a 40 man roster move which isn't ideal
- Keegan Thompson is, aside from Palencia, the guy getting the best results at the moment. But as called out in the game thread his velo is down, and of course he's not a lefty
- Jordan Wicks already seemed relatively likely to come up and provide some innings in short order. That said after going 3 on Sunday, he's probably down until the homestand
- Gavin Hollowell and Jack Neely are the only other optionable guys on the 40 man not covered above
I think the plan probably depends on what the team was already planning with Wicks. If you are wanting to call him up on Friday, maybe you leave Pearson on the roster through the end of the road trip? If Wicks isn't imminent, I would probably eye Thompson or Hughes on the plane with Palencia.