WiFi anyday. From what I have seen on Solwise, you can have probs with mains if the wiring is poor and is it the case that the two adaptors need to be on the same mains ring?
Indeed Wifi doesn't always go a great range, but within range you have so much flexibility.
When you can't get Wifi in the garage just run a length of cat5 out around the garden and plug it into the router, its far cheaper and more reliable for sure.