Doesn't matter in the grand scheme. If one is on your ballot and not the other, vote the easy one. If they're both write ins or both on the ballot pick whichever one you like best. If neither are write ins or on the ballot leave it blank.
At least in the case of the greens, Jill Stein is not running to win president. She is running because she has to run for president in order for down ticket greens to be eligible to be on the ballot. Those down ticket greens could actually win and do sometimes in local elections so that's why she's on the ballot.
PSL is on the ballot for publicity and to spread discussion and conversation about socialist values.
Both of these are valid reasons to be on the ballot and both of these are valid reasons to receive a vote. Both parties are anti genocide and neither will win so it's not too big of a difference either way.