you have to apply based on your permanent residency before marriage ( in this case 5 years).I do not think you have the option of applying either way because in naturalization process , the examining officer reviews your entire A-file to make sure you did not obtain your GC fraudulently and you said you did not get your GC based on marriage.. Since you've obtained your GC before marriage, the proper box to check on N-400 would be: I have been PR for 5 years.
Personally, I do not think that speed of entire process depends on your eligibility for naturalization, but it certainly depends on processing speed of your district office and availability of interviewing officers there.
But I would advise you to research more since I am not a lawyer nor immi expert.
Good luck!!!!!
