I think that you'll come across as more positive if you just detail your product and its features. However, if you can get some of your customers to provide testimonials on how your product is better than a competitor's product, it will look much better than if you came out against your competitor yourself.

If you aren't familiar with Jeffrey Gitomer and his books on sales, I'd highly recommend it. I learned so much about selling through his books when I started my own business. He is the master, and he is big on customer testimonials, particularly video testimonials.

As a startup business, you might first want to check what is legal I don't think I would list the competitors but simply push your product explaining how it is better than all the rest. You might want to target certain marketing groups or ages, if it is legal to do so. I don't know what the product is, therefore, I can not really explain what to do with it.
I think if I were you, I would make sure to detail my product and its features.

I think you need to look at this from the viewpoint of your target customers.

If you are in a new field and they have not heard of any of your competitor then there is no reason for you to talk about them. It won't help your customers, it'll only confuse them, and just make them aware of alternatives that they might not have known about.

On the other hand if you have a new product that is like some very well known existing product, but is much better for your target customer than the established alternative then you probably want to explain why you are better.

Examples...

When I came to Mahalo I did not know much about any other Q&A site, and there was little reason for Mahalo to tell me why it was better than them.

However if someone made a new social network, they would have to make a very good case why I should pay them any attention over and above Facebook and Twitter.

A great example of the latter is Pinboard, which explains exactly its pros and cons versus the much better known Delicious.

http://pinboard.in/switch/

If you want people to switch from something they already use, or choose an unheard of service over a big-name alternative, you'd best explain why.

If you are pioneering some new category, or are in a highly fragmented field where there are no big names, then you don't need to worry about that.

EDIT:

Btw, Pinboard also shows you how to make a comparison without coming over as negative or pushy. You don't have to disrespect the alternatives, you can just make clear what are your strengths, and in what circumstance you are a better choice.