The truth of the matter is that there are only so many minutes to go around. So who gets the minutes? If Marta is available, definitely Marta. She does everything well except for TOs being a little high. The offense flows nicely when she's in the game.
If Marta is out Tess may be a plug and play. We haven't seen her much, but my guess is that she is a better 3p shooter than Marta but not as effective inside on offense, defense, or rebounding. Overall defense is slight advantage Marta and Tess may not direct the offense when it gets stuck like Marta does. Tess may be more conservative in trying to make things happen and have fewer TOs. BUT I wouldn't want to overplay Tess and aggravate her shin splints.
Whatever Tess can't provide in minutes, I'd like to see Rennie put up some shots. Rennie knows her job: don't be a defensive liability, no turnovers, shoot when open. There's no versatility but she can certainly fill a niche, especially when she gets hot.