Deciding whether to side with Joyce Tanner or Dr. Garrow in is a tough decision. Though Garrow's past is murky at best, she was one of the leading scientists in charge of the Oberon project and so has a lot left to tell you. Joyce Tanner, by comparison, doesn't seem to know all that much, but she does promise you a way out that doesn't involve consorting with someone who might be a war criminal.
When you arrive at Casterfell Dam Data Storage at the behest of Doctor Garrow and power up the facility, you'll be faced with the choice of running a command for either Tanner or Garrow, GEN MOVE or GEN ERA respectively. Here's what happens if you choose to side with either and input their command into the facility.
Should you side with [[link]] Joyce Tanner or Doctor Garrow in Atomfall?
After you power up Casterfell [[link]] Dam Data Storage, Joyce Tanner will contact you via the tannoy and ask you to instead run the GEN MOVE command for her at the terminal and extract the data onto a tape. She also says she will no longer help you escape if you follow Garrow's request and use the GEN ERA command. Here's what happens in both cases.
Using GEN ERA for Dr. Garrow
The first outcome of using GEN ERA is that the Casterfell Dam Data Storage wipes all information about Dr. Garrow and it becomes clear she was trying to cover up her involvement in the Oberon project. However, when you return, she'll happily give answers about what Oberon is and explain pretty much everything that happened [[link]] at Windscale.
Joyce Tanner, on the other hand, will be less pleased with you. When you return to her in Data Storage Charlie, she'll confront you and ask why you wiped the data. You can feign ignorance or say you sided with Garrow and she'll accuse you of withholding the data secretly and threaten to kill you for it. If you don't want Joyce to turn hostile you need to tell her you don't have the data and there'd be no point in her killing you.
She'll begrudgingly agree that she still needs your help, which will allow you to keep following her lead and plans for Oberon.
Using GEN MOVE for Joyce Tanner
This transfers all data to a tape that you can collect and deliver to Joyce Tanner in Data Storage Charlie. However, as soon as you re-enter the Interchange, Dr. Garrow will start speaking to you over the tannoy. She'll explain that your involvement with her is at an end and threatens violence if you plan to return to the central processor.
What this violence amounts to is an old lady with a knife trying to shank you—not especially perturbing for someone armed with an arsenal of guns. Essentially, choosing the GEN MOVE for Tanner will turn Dr. Garrow hostile and prevent you from following her lead any further.
Deliver the tape to Joyce Tanner and she'll thank you for your work, explaining that she figured Garrow would outlive her usefulness eventually.