I've had both and in my opinion the difference is night and day between the cheap system and a 'hybrid'. The eBay system is very loud, especially at motorway speeds. After driving for any more than 10 mins it becomes almost unbearable...the novelty of the exhaust soon wears off.
My current setup is 3" downpipe/decat combine with a Milltek non-resonated cat back and it is superb. Almost silent at motorway speeds and every day driving, however put your foot down and the noise is immense, it is in a different league to the eBay special.
I already had the downpipe/decat and the Milltek set me back £350 from Dubmeister, you can pick up downpipes for ~£90.