Both are foam hair dyes in basically the same color. In the end, I decided to go with Prettia because I have used it before, and I liked the description on the box better than the one from The Face Shop.
Here's the contents of the Prettia box:
The instructions are all in Japanese (which I cannot read) but I know the basics are to combine contents, attach foam nozzle, apply to hair, leave for 20-30 minutes and rinse.
Here is my hair color before and after applying Prettia in Rose Tea Brown for about 25 minutes:
Both were taken around the same time of day in the same room with the same light in the room. Although the lighting seems a bit different (I blame my iPhone camera), you can kind of tell that my hair is still dark, but slightly more brown. I thought I would be okay with just one box, but I constantly forget how long my hair is. If you have hair a similar length to mine, I would suggest using 2 boxes of Prettia for sure.
I would have been happy with this color (it's subtle but different than my original color), but I already had the two other boxes of pink brown hair dye from The Face Shop, so rather than return it, I decided I might as well try it!
The next night, I opened the hair dye from The Face Shop:
I used both boxes (since I already had them and I learned my lesson the night before) and it's basically the same procedure - combine contents, apply to hair, leave for 20-30 minutes and rinse!
I'm really pleased with the color I have now, but I haven't been able to get a good picture of it :| Basically it's a nice, dark brown in low lighting (which is great for when I'm at work), but in most other lighting you might be able to tell there are slight pink/red tones in it (see below right)! In direct sunlight, you can really see the extra color (see left photo below).
I know these photos are of horrible quality, and I look super dumb in them, but I had to really try a whole bunch of weird angles and camera positions so that my phone would capture the color -__-. My hair is actually one even color (as far as I can tell), but it might look a bit uneven in these pictures because the light hits one spot more extremely than others.
Ughhh. I'm really irritated that I can't share the actual color with you, but people have seen me in person have told me that they can see the pink/red tones :3
In any case, I do recommend either of these dyes. I'm happy with my hair color. If you're going from virgin black hair, you will get a more mild dark brown color than what's shown on the front of either box.
And if you have long hair, do buy 2 boxes. I think I didn't get the results I was expecting after my first round of Prettia because I was trying to stretch one box's worth of dye for my 2-box-length hair. If you're not sure about your hair length, I would buy 2 and check the return policy from the place you're buying from, open only one box at a time, and then return one if necessary :)
2/27/2014 EDIT!
It's been about 5 months, and I wanted to show how the color has held up.
[These pictures were taken in the same room as the other pictures at around the same time as the first few photos in this post.] [Also, I know my curls are very uneven lol.]
The color looks really obvious and pink-y here but I think it's because I was pretty close to the light source. However, I didn't add any filters, so under certain light, it seems that it will look like this (although the color never looks this obvious to me most of the time)! You can see the contrast between the dyed hair and my dark, black roots at the top. I think that some of the ends are lighter but that's mostly because my hair is more damaged there and not any fault to the dye itself.
Here's a photo where I'm a little farther from the light. You can see that the pink is not so vibrant against the brown, but it's still there. Again, look at the contrast between my black roots and the dyed hair. I feel like my hair appears to be this color or a little darker the majority of the time.
Overall, I'm still happy with the color that resulted from using the Prettia and Face Shop hair color kits :) I wish my hair was naturally this color and that my roots didn't look so obvious and ugly right now but in terms of the products, I still recommend a try!