It's hard, but if I had to pick out of the both, I would pick mascara because they really open up your eyes, especially after out curl the eyelashes.
Eyeliner is good too though to darken and define the shape of the eyes.
But yeah, that's just my personal opinion because sometimes when I don't want to go out without makeup but still want to look descent, I just at least curl my eye lashes and apply mascara and bam I'm ready to go. Really makes a difference.
Both! Combo it up for sure...But if I can only pick one then mascara. I will wear mascara alone when I got to work and nobody can tell I'm wearing makeup. The day I forget to wear it everyone can tell somethings off...So it definitely works for natural looking enhancement
Mascara is better to use. When you apply a lot of mascara after curling your lashes it already looks like you have eyeliner on your upper lash line plus it looks more natural than a flat out obvious line from using eyeliner. I use both mascara and eyeliner for a more dramatic look.
If you do liner correctly it can be very beautiful and enhance the eye or make them stand out. I really don't do much makeup but when I do I a little mascara... maybe once a week. I already have really long, dark, thick, curly lashes so people think that I am already wearing mascara. I would like to try liner but I would probably end up with black liquid in my eye. Them together are great, just don't do too much liner.