Honestly I think both are important. Sadly in the UK there isn't nearly enough education on nutrition in schools which is why we have a growing obesity problem due to lack of knowledge and understanding mixed with misinformation in some media channels.
Portion control is important as well as eating slower. You find many people rush through the food (especially if they are busy) which doesn't give your body time to realise that it's actually full... so you eat more!
Some tips that have worked for me (and it is different for different body types) are cutting down sugar and minimising my carb intake. You get a lot of energy from protein which many people don't realise and associate carbs with 'fuel' so think they need lots of them to function. They can make you bloated and often can be starchy. It causes your insulin to spike then drop which is why sometimes after a heavy carb meal you will feel tired or a lack of energy or sluggish(big bowl of pasta etc).
Honestly if you stick to lean meats (turkey, chicken (skinless, steak) and have veg with lunch and dinner it will just drop off. Fish is great too like salmon, pollock, kippers. You can then have the odd treat here and there as well.
Exercise (short periods of good intensity) is also important if you want to speed the weight loss but it's 80% diet/ 20% exercise. You'll start to feel genuionelly amazing as your insulin levels will be fine, no hunger pangs and hormones balanced as well as bags of energy:-)
Also a golden rule... anything that says 'low fat' or '100% fat free' is generally replaced by tons of sugar so still not good! Go for good fats in olive oil, nuts, oily fish.
I did a hub on High Intensity Interval Training if you wanted some exercise tips too:-) Just need your body for it and no crazy equipment. Hope it helps... could go on for ages about this!