Both are not made with fuel economy in mind, so if that's your concern, look for a AE100 instead.
Hmatic (sonata) is not a bad vehicle if properly looked after, but when it comes to luxury, Cefiro is way ahead. But the HMatic will be at least a good 300K cheaper than the cefiro. On the other hand, XG25, a car related to the sonata but looks much better.
If you want to go for a Cefiro, look for a Japanese made S-Touring. Avoid the Singapore imports.
If you are going for a Sonata, try to find a brand new imported one (tough task).