One of the biggest questions commonly asked. I pursued a K1 back in 2013, but processing was way different back then and we determined it was better for us then, if we did it again, we would most likely pursue a spousal visa. Other factors that are not immigration related may be involved such as: what country is the perspective immigrant from, are they free to travel to a place where they can legally marry easily; same sex couples in a country that does not recognize those unions; the perspective immigrant's life and how easily they can pick up and move to the US; etc. Overall, the CR1/IR1 is a far superior visa overall to the K1, @Crazy Cat has a comprehensive list he can post if he sees this, processing times change all the time, and a K1 may be a few months faster, but in the end as you stated, if your plan is to start a life together, getting married now is a much bigger step toward that goal than getting engaged, and the pursue the immigration stuff afterward.
Good Luck!