A DCF for the CR1/IR1 is more desirable since you don't have to go through all the crap of adjusting from non-immigrant to immigrant status once in the USA.
Your wife could not have an indefinite leave to remain visa if she'd been in the UK only six months.
Instead of asking us here what London's residency requirements are for DCF, I recommend going straight to the Embassy and asking them yourself. Looking at the updated I-130 checklist there does not appear to be a minimum time requirement but I do not believe she'd be able to petition for you if she was on a tourist visa, which I don't know if she is or not; there is no such visa as 'indefinite leave to enter' so I don't know exactly what her immigration/residency status is in the UK. CHECK WITH THE EMBASSY.
This is what an indefinite leave to remain UK visa looks like:

This is what a limited leave to remain UK visa looks like:

This is what a UK fiance visa looks like:

This is a UK tourist stamp:

Which visa is she on? It'll probably matter.