Affiliate links are preferred over adverts, but they should always be disclosed. Affiliate links are like adverts, you're getting money to advertise a product, so while adverts outside your article don't affect the contents affiliate links do -- even if you refuse to have any bias.

It's just good manners to always mention that the links are affiliate links because it can change the value of your review and/or recommendations when money is involved and it should be the readers choice. Just like if you're reviewing a book you wrote, or a company you work for.