I think the weight difference in this case Onkyo vs Denon can be attributed to the fact that the 929 offers 9 channels of amplification while the X4000 offers 7 with the ability to process 9.
As to the initial point:
Your current setup is 7.1 so either receiver will be able to do the job competently. As for the HDMI issues of past Onkyo's, I would not be concerned as I have year to read about issues with the HDMI boards on the '2' series (i.e. 909 / 818 etc)
As to what is best for you, really depends on how much you are going to grow into the features of the receiver. Having Sub EQ on the Denon is great, but if you are not going to be using 2 subs, no use for it. Having 9 channels of amplification might work great if you want to have a Zone 2 and don't want to take away channels from your zone 1. Then again, if you don't need it, its not going to be a huge selling point.
My additional ramblings;
I have Integra, which is basically an Onkyo. I have had the 80.3 Pre/Pro for about a year with 0 issues and have had the DTR 30.5 for about 6 months with zero issues. I will point out that the HDMI board on the 30.5 gets warm, but no so much that you cant touch the grill.
Denon is a great receiver which comes with a better warranty and some unique features. I just was never a huge fan of the looks of the unit, which is totally the wrong reason not to buy it
Hometheater.com gave the 828 Onkyo, not so great reviews on the audio, which I find funny because so many hear say that all amps sound the same, but this article goes on to point out the bightness of the Onkyo, where it is essentially the same amp that received all positive reviews in the past.