Well, some new information regarding my experience with the two programms.
About three days ago my wife accidentally upgraded from Avast Free to Avast Pro Trial. I suppose it's it was one of those silly tricks where "No, I definitely want to stay with the free version" is written in very small gray letters and you just hit the big colourful "Yes, I want to UPDATE" button whithout realizing what the result is going to be.
So, I've done the following:
1. Download Avast removal utility.
2. Run in Safe Mode.
3. Uninstall Avast Pro with the said utility.
4. Reboot in Safe Mode.
5. Uninstall Comodo Firewall (and D+).
6. Reboot.
7. Install Avast Free (latest version) without the Network Shield.
8. Reboot.
9. Install Comodo 6.0.264710.2708.
10. Configure mutual exclusions (in Avast and Comodo - safe processes, executables, shell code injections, trusted applications etc, etc).
BTW, Comodo now has no Defense+ but a HIPS section. They've also "upgraded" the interface - now you can't see your current connections and you'll have to thouroughly search before you find the settings (they're really inconspicuos). It may be due to the fact that I've chosen an option during Comodo installation stating that I want Comodo not to bug me with a lot of decision-making stuff. However, I thought it will merely effect the basic settings upon Comodo startup - Safe mode and so on.
Anyway, what I've got is a computer that will boot in about 10 minutes. I mean it becomes somewhat responsive after this interval. Depending on how early you try to run programms it will hang up or run real real slow. Browsers start in 1,5-2 minutes. Page load times are horric when they're sucessfull. Sometimes they just cannot load.
I've done some more fiddling with the settings. It turned out Comodo has added tons of new executables which I didn't notice at once (I've done the whole exclusion thing virtually hundreds of times and remembered the necessary files). Ok, add them to Avast exclusions as well. Now the system is working more or less responsively although load times are still not brilliant.
Finally, I turn HIPS off. Now everything's as it should be.
To conclude: my impression is that either the Comodo and Avast are still not working seamlessly (at least not on older machines without 4 gigs of RAM and some serious CPUs) or the new HIPS is a serious performance hog which is a shame of course.
My system is Athlon 3500, 1.5GB RAM, WinXP Pro.