I am not up to date with the latest processing times, but historically it has been a bit faster to get the K1 than an immigrant visa. However, and it’s a big however, if she arrives on K1 she then has to undergo adjustment of status after marriage, which depending on field office could take somewhere up to two years to get a green card, she would need to apply for a work permit and advance parole if she wants to work or travel outside the US (those can take some months to get), and in some states those who entered on a K1 awaiting adjustment have reported problems with things like getting a drivers license (as technically their visa, one time use, has expired). If she comes in on an immigrant visa she is a permanent resident/green card holder as soon as immigration stamps her passport, can work and travel immediately etc. so I don’t know about “safer”, but it is definitely a lot easier after arrival to do so with a spousal immigrant visa rather than a K1.