I think either are of equal safety. As in, both incredibly safe in the grand scheme of it. J&J was one out of every 84k women of a certain age range getting a severe blood clot, but that's been all they've really found as a true indicator of safety.
Both vaccines create the same spiked protein. Moderna/Pfizer used mRNA to produce it, J&J uses an adenovirus. But the actual vaccine is essentially out of your system in a few days, and now it's just about memory. For all intents and purposes, any negative long term effects would almost assuredly be seen within a couple of weeks, and traditional vaccines of the past have all seen negative effects within 6-8 weeks. There's really no reason to wait 4, 5, years for these effects. They would almost certainly start to rear there ugly heads sooner. It's why Pfizer might be approved outright this month, after just a year. In reality, these have been under more scrutiny and followed publicly more than any vaccine in history. The fact that there are so few issues is incredible.
J&J is likely just as effective as the others. It's about 72% initially(after a couple of days), and then believe to be up over 90% after a few weeks. It gets stronger, and it was tested in a later time of the pandemic working against many other variants. Basically the same rate of prevention of death(as close to zero as realistically possible) and hospitalizations.
https://www.chop.edu/news/long-term-side-effects-covid-19-vaccine