Obviously, you're in a CodeIgniter forum so the answers you get might be biased. Both are very good frameworks. So the answer is, unfortunately, it depends on your project.
- You'll be able to find more developers for CodeIgniter, which might be important if you need a team.
- The developers you find find for Phalcon MAY be more experienced developers, though that's not dissing CI devs at all, just the reality when you have a large pool of devs for a very easy to pickup and get running framework like CI.
- Phalcon will work great IF you have control over the hardware or VM used to run it on. Since it's an extension to PHP itself, many shared hosts won't let you run it. If this a software to sell, this could be a deal breaker since you want things easier for your customers. If it's SaaS you will have control so the extra speed boost might be worth it.
- If the details of Phalcon will need a lot of customization then your performance will drop as you customize functionality, unless you have C developers on your team with experience writing extensions.
You said the project is a price comparison site? If that's the case it probably doesn't matter which you choose. Which is more comfortable for you and your team? In this case your database queries and structure are going to be your biggest performance hits, as is the case with most sites, honestly.
All that being said, if you haven't used Phalcon before and the parameters are right to allow it's use in the site and you just feel like giving it a test run, then go for it! The only way to truly learn the differences and what you like/don't like in the different frameworks out there is to use them in real projects.
