I wouldn't call either a plain "stereo integrated.". Both of those are receivers, except the Rotel has an AM/FM receiver built in while the Onkyo is a digital (input) receiver. Stereo integrated is more like the NAD 3020, digital stereo receiver is more like the NAD D3020 (the new, tiny one with USB input and sub out).
Depends on how you'd hook things up. Go with the Onkyo if you can use SPDIF to hook up to it, that way digital audio goes in and the analogue signal would have a shorter signal path.
Amplifier technology isn't all that drastically different in terms of how different the performance is. What you need to take into account are all the digital stage features, like USB inputs, Network player LAN input, etc, when/where applicable (also dedicated subwoofer output with crossover on newer Network stereo receivers). In the same manner this is why you see HT people upgrading a lot more than 2ch stereo listeners who by some point have an amp that won't distort with practically any speaker they throw at it getting loud enough in their rooms, like getting into using HDMI or going from HDMI1.0 to a later version to simplify connections. If anything, when it comes to HT receivers, amp stages and power supply designs seem to have improved on entry level units (likely a response to affordable performance from Emotiva).
In your case if you're just going to use analogue inputs just look into the performance of the amplifier stage, as long as you can otherwise have all the analogue inputs and outputs you need. If anything, the prospect of the older unit busting anything far out of warranty and you have no store near you to fix it (and shipping heavy equipment like this is going to cost a lot) should be the deciding factor as far as age is concerned.
