It is up for debate whether Ayn Rand could have taken the money that she'd have gotten back in her SS checks and have done better with investing that money or not.
She certainly didn't do better with the money that wasn't taxed (or she wouldn't have needed social security in the first place). So why would we expect her to have done better with the money that was taxed?
Furthermore, she, who was supposed to be the pinnacle of personal responsibility, failed to be responsible for her own life. How can we expect those who have fewer opportunities that her, to be more responsible than her?
That's like saying that someone who opposed Communism that waited in a food line in the Soviet Union was hypocritical for taking food from the government while opposing it.
The difference is that Ayn Rand railed against the program for years, and in the end she needed it. It's not hypocrisy that she took the money that undermines her entire credo. It's the hypocrisy that she claimed no one truly needed the safety nets of society, that only parasites would use them, that it was easy to live your life without ever needing to use them. The hypocrisy is that after spending years claiming no one should need them and having been given every opportunity to ensure that she did not need them, she failed to live up to her own minimum standards. She failed to do what she had declared was not only simple, but the duty of every American. She failed to stand on their own two feet. She, who had so much more opportunity for success than so many of her fellows, was not able to do what she claimed everyone should be able to do. So, in the end, everything she claimed and stood for was exposed as arrogance and wishful thinking.
Safety nets exist because even good people can stumble and fall, and it's a shame that Ayn Rand was never able to understand and admit that. She was so wrapped up in her bolsephobia that she was never able to see the government in a rational light. It is a bigger shame that she has a legion of parrots who look only to her ideas and ignore her reality because it suits their wishful thinking to do so.