Connect both HDMI and component then see for yourself which is better in your particular set-up. You will likely see a difference. Which is better can be quite subjective. I find the digital connection (HDMI) sharper and the analog (component) "warmer" with more color depth. It also varies with program content. Compare also to the composite/S-video connections for a real hoot!
You might want to keep component connections intact even if you go with HDMI as your preference, so you'll "be prepared" if/when the HDMI output goes dead, as has happened for many including myself.
IMHO, the "HD" installation is not complete unless the installer connects at least the component outputs, and that would require him to have those cables with him. Anything less is not HD and thus should fail as a complete installation! I would have at least component cables available at the time of installation. Don't offer them unless it's clear that the installer won't provide a set. You can get component cables fairly inexpensively from several sources including Wal*Mart. Don't spring big bucks for them, expecially not for the HDMI cable. Both types are available on-line for as little as $10 each set, maybe even less. Don't forget that with component connections you will also need L+R audio connections. If you have a separate audio system you will also need the Toslink (optical) cable for the digital audio. Check the monoprice link Poke provided. There are many others...