Immigration to USA (marrying a US citizen)

Hello  :)
I'm new to this forum, please excuse my ignorance on most matters!!
I'm looking for advice and knowledge on the visa/immigration process.
I'm a British citizen, and my partner is American. We both currently reside in our respective home countries, and would like to start the immigration process soon. We intend to get married, and this is where I need advice!
Based on your experiences, would you say it's best to go for the fiancée visa or spousal?
I can't make head nor tail of which option would be ‘better' and most likely to ‘go smoothly'...we want to get it right. Also, I have two children who would be moving with me. My eldest turns 18 this summer, so from what I can find online she wouldn't be able to apply for permanent residency in the same way as my younger child (currently 12)?? I'm very confused, so any advice would be greatly appreciated. Thank you so much! :)

Please do not use what you find on the Internet. There are two official sites which are interlinked. Uscis.gov and travel.state.gov.
You do not have time for K1 if you plan on bringing both children to the US. https://www.uscis.gov/family/family-us- … -residents

Thank you! So in short, because my daughter will turn 18 before we begin the official application process, we will need to apply for her VISA entirely separately? So it'd be better for us to go down the fiancé VISA route rather than spousal?

https://travel.state.gov/content/travel … -2020.html
Visa processing times and availability for children/family members. Bulletin gets updated every month. There is no "we".  You can sponsor her.
Your CR1 spousal visa has to be in process before she turns 18 or she will stay in the UK unless she comes up with her own visa. Student will be an option for a couple of years and that depends on her education and your check book. You do not have the time to go K1.