I have the 176C and I contemplated the same things. What I ended up doing was pruchasing the pre-loaded cards because I found out that you also should get the USB writer along with the CD or it will take a LONG time to download to the unit. That along with the need to purchase blank cards to strore the BlueChart data raised the bar to the level of the pre-purchased cards (actually they were cheaper). From what the Garmin people told me, the only big reason to go for the CD is if you want to plan and track your trips on you computer in addition to the GPS.
Hope that helps.