You can only decide for yourself. There are a few reasons why the two might sound different:
- Bitrate
- Mastering
- Placebo
Bitrate probably isn't going to make a difference if your iTunes downloads are 256 kbps, but some people can hear the difference if they try. Some songs will have more noticeable compression artifacts, so it depends on the individual CD as well. If your files are 128 kbps, it may be worth seeking out CDs, but test yourself first. There are several tests floating around between 128 and 320 kbps MP3 files, as well as some comparisons to FLAC.
Mastering will matter only on an individual CD basis, and probably with a lot of modern music both the CD and iTunes use the same mastering. This is the most significant difference, if there's a difference at all, and you'd have to do research into the CDs to determine if it will matter.
Placebo isn't going to cause a real change in sound, but without blind testing you'll probably think there's a difference. That's not a bad thing as long as you aren't paying out the butt for it. Simply put, if you'd rather buy CDs, then buy CDs because they'll probably "sound" better if only because you like them. When people describe the intricate differences between high bitrate lossy and lossless, this is usually what they're describing.
Then there's the other material advantages to both formats, that have nothing to do with sound. Obviously the iTunes files take up less space, and you might like how they integrate with iTunes. The ripped CD files will be a perfect replication of the data, which is nice to have, they aren't tied down by DRM, and you can re-encode them to lossy formats whenever you want for portable use without having to worry about additional degradation (encoding lossy from lossy results in more damage to the data). You also get a nifty physical reminder that you own the music, and you can sell it later on if you decide you don't like it anymore.
Me personally, I will never buy digital lossy music, and I don't want to buy digital lossless music either. I want a perfect copy of the audio if I'm going to be spending money, and I prefer a physical copy too. I can't hear the difference, but the sound isn't why I spend more on a disc.