When left second bar, the fuel light starts showing. But will have no problems driving until zero bars as the reserve/unmeasured portion is about 5 litres.

I can average about 650km per 45L of RON95 petrol which is about 6.9L/100km or 14.km/L.

Does your place have a significant uphill climbs or got quite a lot of need to do overtaking? But I don't really feather footed and rather ah beng in early mornings and I still can get that fuel economy....
If your referring to the new Civic with the new 1.5L turbo engine :

Technically, fuel economy will favour the Civic, but not sure in real life. My guess is should be similar.

Maintenance cost will be Mazda as fuss free for the 3 years.

Ease of headache, both cars won't cure your headaches (just kidding  ). So far not heard of any bad cases of engine and gearbox failure for the Skyactiv petrol engines. The powertrain is consider rather reliable from a UK car website and have good recommendation for Skyactivs. The common issue with the Mazda 3 being :
1) Noisy suspension even when new (most of us complain to SC kaw kaw)
2) Loud tik/tak noise on windscreen for some, a little annoying but less frequent than the suspension noise
3) The NVH insulation of the Mazda3 will really need to step up on the upcoming facelift, the factory fitted performance tyres made this worse (the new Civic will beat it hands down)
4) Some complain of being a little tight at the rear seats. Will be an issue for tall people, but my family members are fine with it (new Civic wins this hands down as the space can compete with the Accord  )
5) Skyactiv engine roar vs Vtec scream (personal preference applicable)
6) Some with light tickling/rattling of the dashboard
6) Brake feel and travel not so nice (personal preference applicable), but the bite is absolutely adequate even when needed to slow down from high speeds.
7) The "Mazda" brand vs the "Honda" brand (suka hati kau la.....)

In the end, your choice la...despite continental cars having vocal complaints, there are many people still flock to them as the emotional appeal can be stronger than rational appeal (example : there are more W205 C Class on the road than the Mazda3 within the places i drive often). Get what satisfy you most. If you're in the lookout and not in hurry, please do wait for the new Civic and the new Elantra later on (i have a 2010 hyundai back home and it is rather solid till this day with no nightmares other than the regular service).