The initial paperwork load for the actual visas I would say are very similar.....obviously you would be filling out different forms, but I would say that the evidence you need to gather and the actual time it takes to assemble all the documents is similar. HOWEVER, as I said in my previous post, the big difference is the AOS paperwork. This is a lot of work...with the K3 this needs to be completed once the spouse has entered the US. If the spouse enters with an immigrant visa, then AOS does not need to be filed, therefore eliminating this paperwork all together.
As for the cost, I believe the actual immigrant visa is initially more costly than the K3...HOWEVER, the K3 holder will then have to pay $1010 to process AOS.....this fee alone is more expensive than the immigrant visa (Im not sure of the exact cost of the 2 visas), therefore in total, the K3 visa works out significantly more expensive.
Im having trouble understanding what you are asking in your last sentence.....are you asking if they get divorced before the interview what would happen to the wife?? If so, then if divorce happens before the interview then the visa petition is voided as the visa is for a spouse and therefore they both must be married....if one is divorced then one can no longer proceed with a spousal visa application.
If they divorce after the interview and the non us citizen is now in the states, then if they entered on an immigrant visa, the non us citizen can apply to have the conditions removed from their greencard 2 years later providing they can prove that the marriage was entered into in good faith (ie, not a marriage carried out purely for immigration benefits). Please correct me if I have understood your last question wrongly.