For me eating healthier is harder, exercising more is definitely easier!

I just love fatty food;) but I know what I eat & I always excersise, not because I've to but because

exercising is so much fun.

I think both are important to get healthy, but the most difficult is to stop eating junk food.

There are so many ways to exercise alone it doesn't need to be difficult.

"Eat healthier" and "Eat healthy" is how it is said colloquially in the US Moira. It's all over the media and how we often say it with each other. Thank you though for pointing out that it isn't really grammatically correct. Eating well and Eating better is how I most often express the idea though and I believe that is both grammatically and colloquially correct so that's an option too. 

Eat more healthily. (not healthier)