Bre, this is my input. Of all the colors, red is the hardest to keep in your hair. Meaning it washes out the fastest. In order to get the red to show up, they will probably have to bleach it first, so it might take some upkeep to keep the highlights looking fresh and red.
Henna is different because it really is permanent, especially the more treatments you do. I've been using henna for about 4 years now and the whole length has taken on a red tone, especially in the sun. It didn't take 4 years to get that way, but probably a few treatments.
Another thing to look out for is the loosening of your curl pattern. I had some blonde highlights put in years ago and the pieces that were highlighted weren't as curly as the rest anymore. Bleach acts like a texturizer so watch out for that. I would probably go to a professional like Curlee said, but I have had some "professionals" who messed up my hair BAD so be careful. The time that I really liked my highlights, I went to a professional colorist, someone who ONLY does color, so she was really good. She was in Arizona though, or I just might go back.
Henna can loosen your curl pattern too, but only temporarily. I haven't seen a permanent loosening of my curls in all the years I've been using it, it only lasts for a few washes. AND it's actually good for your hair. My vote is for henna!
Anyhoo, that is my input. Not to scare you but just trying to help you make the best, most informed decision :Rose: