As long as you incorporate a post-workout meal, I think both are equally effective for preserving muscle. What it comes down to is convenience (at least for me). For the last several months I'd fast every other day from 7-8pm until about 5pm the following day (~20hr fast). This worked well b/c most days I did not have to worry about preparing food for the work day or even spend time thinking about food. I'd workout around 4:30pm-5, ate a decent size meal about a half hr later and call it a night (still going to bed on an empty tank for optimal gh release).
Just started experimenting with the 12am-6pm window. This seems to be slightly more effective for fat-loss since you get to expend some am cals while working with a faster metabolism (it is naturally faster in the am) and then get the benefits of fasting everyday for ~18hrs or so, as opposed to eod.
So it really boils down to preference in my opinion with the 12-6 window tipping the fat-loss scale while a pwo meal addresses any muscle loss concerns one might have.
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