I would like to seem a fact supporting this, I have never been to a country where comparing objects in ads was outlawed, but to be honest I have spent most of my life in the US and travled only to Western Europe.
Spain is one of them. I cannot quote the law but simple experience shows it.
Edit: there is some EU law about it [1] (it was bound to be...).
In the end, as you can only compare 'objetively', you will never do it (because your advert will become boring, mostly). So jokes like Microsoft does about Google are outlawed.
Here, in France, it has been outlawed forever until it was finally authorized in the early 90s. As a then advertising student I can remember there was quite a fuss about it. In reality, comparative advertising is restrained by relatively clear rules and very rare in the wild. The few occurrences of side-by-side products I seem to remember actually involve large US-based international companies.
Usually, the comparison part is limited to a features table.
