have used both, the all in ones are great for starting out, but the filters are kinda pathetic and the carbon pads are a money waster. got a 7 year old AR980 at work with no problems so I don't think the build quality is bad. and the modifications of the filter to a external are easy as. My main gripe with them is the lights are normally right at the front so makes planted setup a bit harder (especially when you want tall, light intensive plants as a background.) I'm quite impressed with the new Aqua one nano cubes though, the built in sump filter looks quite good, anyone had any experience with these????
personally I prefer to build myself from a bare glass tank, that way you can get the best equipment for the job, and not stuck to just one brand.