Depreciation is the most expensive part of owning a car. The largest depreciation will hit the first year, and keep in mind that very few pay sticker price, especially with GM who seems to be willing to go even below "invoice". Basing used price on sticker is a bad idea, it needs to be on what are people actually paying for the cars. If the dealer made larger profit over the average amount spent then thats all part of the loss of buying. When I enter in my car to KBB, with 6k on the clock the private party value in excellent condition is roughly what I paid for the car new....In reality I still expect about a 10-20% loss the first year. Don't forget the money spent on taxes.....
Honda doesn't (have to) discount as heavily...this actually helps keep the price stable.
Nobody really knows yet, but the projected resale of the Sonic is not bad, (although still not as good as the Fit). If the Sonic ends up aging well that will help to increase the resale, but GM subcompacts are not going to be top of the "want to buy" used list for a while. However, affordable cars with good utility and good gas mileage will not depreciate as fast as others due to high demand in that market.