Essentially the faster the wifi, the shorter the effective range. Often I'd blame the receiving computer rather than the wifi but Macbooks of all varieties have very good wifi kit built in.
Can I assume you live in an area with lots of other houses? There are only so many wifi channels available as it wasn't designed for use in areas where more than 10 wifi networks might be competing for the same aerial bandwidth. Also, wifi is quite intolerant to old-fashioned heavy brick and stonework houses, if you're using the latest wifi technology then it's even worse. We live in an area with very few competing wifi sources and our house isn't that big but I still have two wifi routers working at once. I deliberately have them working as two discreet networks, one connects the wifi infrastructure of the house, e.g. server to AppleTV, radio, etc., the other is my normal usage one for our laptops, iPad and phones, that way I can stream big files on one while still getting full speed on the other.
I used to use the powerline stuff here at home when wifi didn't go above 54mbps, quite good but effectively you halve the rated speed for every segment of your home electrics you're using. For example, if your internet router is on the ground floor ring and your computer is on the first floor ring then you should really expect no better than 1/4 the rated speed. This is entirely dependent on the quality of your home electrics, if you're using chunky premium home power cabling then you'll go higher, if you're using bargain basement cable using a few strands of copper then it'll be abysmal. Also, this is very disruption prone and also disruptive, put your microwave or hairdryer on and you can say goodbye to any reasonable data, some people I know even lose data speeds when their boiler kicks in. On the disruptive side, if you have a neighbour that uses CB radio then they'll not appreciate you at all as you'll be seriously interfering with their signal. That all said, it's a very good way to connect isolated bits of kit that have no wifi, e.g. SkyHD boxes, and can be a neat solution to fixing data connection problems in old houses. p.s. you can usually ignore the stuff about not plugging powerline kit into extension cables, that's not very accurate, it should be a warning that if you have high power drain kit on the same extension cable then you run the risk of data degradation but general usage equipment is usually fine.
When we were getting fibre installed I realised that the powerline stuff just couldn't handle it effectively and ran a single CAT6 cable from my front hall internet router to my office upstairs. Took a bit of careful planning to make sure it was neat and hidden but it was definitely the best decision as it's full speed and essentially interference free.