When a company has a product it means they have it to develop. In such case, a company shows why they need ICO and how the company will be growing. What is more, there is more trust because a company has experience in a field they are working.
For example, KickCity company that exists two years. KickCity an event platform that generates revenue and has a Helsinki government grant. KickCity needs ICO to make business bigger and to become a decentralized platform where every user is rewarded for ticket promotion.
On the other hand, no product ICOs can prove their success just in theory. There is no proof base that product will be developed and will be working right.
In such unstable industry as ICO everything has to be under control. The product is one of the things that shows the campaign is not a scam. That is why I believe it is better to invest in ready-made products ICOs.
I also try to focus more on project with existing product, but not all projects with existing product are winners...sometimes it is companies that make no money and are getting bankrupt, then they raise money with ICO to keep going...