Personally, I think Chi would be better. It looks much like GHD's, which in my opinion are amazing. They work really well. But just look for ones that heat up pretty quickly and efficiently. If you have thick hair, just separate it off to straighten it, don't worry too much about the heat per se <3
According to reviews, HerStyler is far much better than Chi Hair Straightener. This is because it is said to be efficient and works well for any type of hair. Get in touch with a professional stylist for more advice.
I have a Chi and I love it. I have extremely frizzy/fluffy hair and with the Chi i only have to go over each section once, occasionally twice, and it straightens the hair perfectly. Also cause of the thinness of the plate, you can use it to curl, flip, and straighten which reallllly makes life easy, especially if you have hair like mine because instead of straightening and then curling, you can just curl & straighten at the same time and save yourself some time and heat exposure. Also, it fully heats up in less than a minute which SAVES YOU TIME. The preset heat temp is perfect for getting the job done and also saving your hair a little bit of damage. Plus with just one switch on the straightener, you don't have to "figure it out" its reallllly easy and actually pretty fun to use. I love it and I totally recommend it.