I have tested the a7 extensively. Speaking of the original a7 and a7ii, no serious advantage concerning potential image quality, even while using the same lenses. In fact I've made a comparison using the same legacy manual lenses, with the Fuji utilizing a focal reducer.
The a7ii has IBIS, which is a considerable advantage. OTOH, if you consider available Fuji lenses vs available (native) Sony ones, the end result, IMHO, favors Fuji. Not to mention that cost is higher for a similar a7 system. Personally I also find the look of the Fuji output much more pleasant and "organic" looking.
The a7R and, of course, a7Rii are another matter. If you need high pixel count the a7R is a good bet, albeit with serious quirks, such as shutter shock. The a7Rii seems to be a superb, albeit expensive, imaging tool, which I myself will seriously consider when lens line-up is a tad more mature.
I'm not trying to belittle Sonys and I'm far from a fanboy (in fact, I use them too, as well as Olympus and Panasonic cameras). For the time being, it is a somewhat limited system, very good for adapted lenses and for those that are ok with the current lens ecosystem.