The obvious answer would be to keep both and see what you think and then flog off the hi-fi if you were happy with the Onkyo.
But I'm guessing you want to use the money to put towards the Onkyo.
Hmm, a £300 hifi, which will probably have a better amp section, but has been compromised to fit all the other bits in for the price, or a receiver that has been compromised to keep price down?
It depends how critical you are, but I'd guess it would give it a run for the money. Just a guess though.