The k-1 only allots 3 month visa, not a green card. If you change your mind and don't want to marry, the k-1 visa holder can't change status to another visa, cannot extend their stay, and cannot adjust status (apply for a green card based on a different sponsoring spouse or based on a different family member or employer). Even if the k-1 later marries another US citizen, the k-1 will have to travel abroad to interview at the consulate abroad. This can many times lead to the ten year bar and the extensive waiver process because of unlawful presence.
The k-1 may be a little faster than an immigrant visa based on marriage but the downside is that the K-1 does not allow the immigrant fiance to enter the US with a green card. Marrying and going through the immigrant visa route and consular processing based on that and not the fiance visa route, would allow the immigrant to come into the US with a green card already, not a temporary visa. The trade off is the longer processing time, usually as little as an additional 3-4 month wait. If the fiance entered on a k-1, the immigrant fiance would have to go through adjustment of status process once they marry in the US to obtain their green card, which costs more in filing fees and takes longer for the immigrant to receive a green card and requires the couple to interview for the case. There are pros and cons to filing as married or as engaged. Timing, money, and immigration history can have a major play in the right decision for your case.
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