Provided the ceiling plasterboards aren't foil backed, in the loft central ish over the building outline is less obtrusive.
I bet that most peoples trouble with WiFi coverage is down to poor placement, RF wise, of the AP. Mainly beacuse of the "one box" modem/router/switch/AP solution. It gets plonked within 6' of a phone socket and shoved somewhere out of the way so ends up in far flung corner of the house and partly buried under other stuff.
Even better if you have it on a different channel, so both APs can talk at the same time. Why do you have a different SSID? To get coverage here I've two APS, on different channels but same SSID, devices seamlessly switch between them.