Hard candy have a better track record for me. ELF tends to dry up or break up whatever the case and such is not the case with hard candy. i ran out of mascara(it is no less product than other brands) before it dried up which i cant even say for some HIGH end ones
I find that these are both great brands but you do get what you pay for. I think hard candy has better products. Elf has some great products too, don't get me wrong, but I prefer the quality of Hard Candy.
I don't wear make up too often, but I do like Hard Candy's eyeshadows and nail polish. The colors are bolder and brighter. So, if that's the look you're going for, stick to hard candy :) I've found elf is more natural.
Personally I love hard candy and ELF. I tend to find that some products are great and some are not with both but as for shadows, I am loving elf. Reason being is that pigmentation is not so great in some of the palettes that I have by hard candy. Hope this helps.