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.
The obvious question is what is the £300 hifi you have. Is it a midi system type thing or an actual amp and speakers.
If its the latter then the hifi will sound better for music no question. If you got the Onkyo though you could still use that amp to power the front 2 and just let the Onky do the processing, this would lead to the best results all round, but it depends on what your trying to achieve.