for k1 you need 100% of poverty line and assets need to be 5x the income req, think just over 100k now?
spousal is 125% and assets need to be 3x
Your parents will be cosponsors, so you'll both fill out an i-134 (or I-864 if you go spousal)
I am not up to date on spousal or if you can do consular processing but I think spousal takes about 12-18months on ave in London
Fiance is taking 6-8months
Spousal is the better financially and giving your partner right to work/drive etc but if time is main priority, which it may be here with parents being ill, K1 may be better. As I say, not up to date on spousal route so I could be wrong but 'what is fastest' is not usually what people consider when advising here, for me that was a big factor which is why I did k1.