I would be very surprised if you could detect a convincing difference between the SQ from these two receivers. Both should do a very good job and the delay from switching between them is going to lead to uncertainty of any nuances you might think you heard.
Really, I believe the decision between these two units is more about features and usability. Don't forget to evaluate the other RC (remote control) unless you use a universal remote.
As far as upgrading goes, I would probably return both units if it is not too much hassle. Essentially, you just realized an upgrade to your receiver. Enjoy it and consider if you've got a better place to put that money.
On the other hand, NEW RECEIVER!
Edit: I just saw your other thread and it sounds like you will definitely be upgrading!