Unless you're deaf as a post or you're using inferior ear buds (read: Apple ear buds) then there is a vast difference in sound quality between the OGG files from Spotify and AAC lossless files in iTunes.
Thats not a statement of opinion, it's fact and whilst I appreciate that not everybody will always tell the difference, if you're playing these formats through a decent hifi or headphones then you should notice a big difference.
Don't get me wrong, I use Spotify a lot and pay for the premium service and it's not bad at all, however if I sit and listen to it on my hifi at home it makes me cringe because of how flat and compressed it sounds compared to higher quality formats.
If you can't tell the difference then it really makes no odds, but there is a difference and plenty of people can hear it which makes it a fact.