With a HTiB system, the speakers are almost always the weakest link - which is another reason why having an upgrade path is so important.
The speakers in the Onkyo 5300 system aren't great:
- They have slightly below average sensitivity (85 and 86dB for fronts and center) - good sensitivity is closer to 90dB, and is a measure of how efficient the speakers are, or how easy they are to produce high volume levels.
- They are 6 ohm speakers, which is about as low as you want to go with a mainstream receiver designed to use 8 ohm speakers (like the Onkyo)
But they are still, IMO, far superior to the Sony speakers (where the sensitivity and frequency response numbers aren't disclosed at all...):
Comparing the subwoofers --
- Onkyo has a 10" subwoofer, Sony has a 6.5" subwoofer. Bigger is always better when it comes to subwoofers (and technically, 6.5" is not a subwoofer).
- Onkyo sub has 290W of power vs. 165W for Sony (at nearly double the wattage, this is a meaningful difference, and when it comes to subwoofers, wattage means a bit more than it does for the rest of the system.
- Onkyo sub reaches down to 25Hz, Sony sub reaches, well, they don't list that in the specs frankly, because a 6.5" driver isn't going to get anywhere near 25Hz.
Rest of the speakers:
- Onkyo mains have 5.5" drivers plus a 1" tweeter, Sony mains have what looks to be a single 3.25" driver. It's really hard to have a flat frequency response with a single driver.
Of course, you do bring up a good point - it does depend a great deal on how they actually sound. The only way to know which is best to you is to listen to them. We all hear things differently, and you can only trust the numbers so far. They tell only part of the picture, and a theoretical one at that.
Honestly, I would be pretty surprised if the Sony speakers were to outperform the Onkyo speakers in a head-to-head comparison, but that's not to say it's beyond the realm of the possible.
However, I keep coming back to the upgrade path - at least with the Onkyo system if you decide you don't like the speakers, or need something more, you can actually go out and get something to replace the stock speakers (even do so one speaker at a time, if needed). With the Sony system, you'd be out the whole shebang and back to square one.