I happen to prefer traditional pressing combs over the kind you plug in. Believe it or not, you have more control over the amount of heat you put in your hair. Yes the electric pressing combs have control dials, however the heat has been "predetermined" for you, so that means sometimes you may get too little or too much heat depending on the setting you choose. In order for me to get my hair straight using an electric pressing comb, I have to use the highest setting on those things, and that is way too hot for my hair, yet the setting down from that, isn't hot enough.
With a traditional pressing comb, you have complete control over the amount of heat...so you can get a nice full big fluffy press, or leaving it in the stove a bit longer will yield a sleeker smoother straighter press...you could even do a "textured press" where you have that big Diana Ross look, the hair is still straight, but it has more texture to it. It is much harder to do this (in my opinion) with the electric comb given the preset temps.
Second reason I prefer the traditional is because of the heat source, the traditional combs are placed in a thermal oven, which yields different results when you press. If you use thermal straightening techniques correctly, there is a distinct difference between softness, and body when using a traditional combs, that electric combs can't even begin replicate because of the heat source.
Not to mention, they can be more drying/damaging than traditional combs placed in thermal ovens. However improper use of traditional combs can be just as damaging if you aren't careful, a perfect example would be using a traditional pressing comb on the kitchen stove, if done regularly that can really take a toll on your hair. I am saying this to say; All heat sources are not created equal. - They have definitely made strides with electrical equipment, with the invention of ionic technology (ionic products like the chi..etc) and this was due in part to the extreme drying effects of a lot of plug-in hair devices.
If you plan to press your hair on a regular basis, I would suggest you lean towards a traditional comb, and invest in a Golden Supreme Heat Tester (I did a review of it in my blog, you can find the addy in my fotki). This way you can ENSURE that you NEVER have the comb too hot, and you can enjoy the benefits of using a traditional pressing comb (softness, body, shine, etc).
My ALL TIME favorite combs are Kizure, the heavier the comb the better, I love those combs because they have even heat distribution, and they hold heat well. Even heat distribution is key to make sure you can press your hair straight in one pass. They sell them at most Professional Beauty Supplies, I don't think I have seen Kizure combs at Sally's, although it may differ by region. In my opinion they are the best you can get. You can order them online through the company
here
They have pressing combs for fine hair, super coarse hair etc, the temple/hairline, and even a double heavyweight comb.
Sorry this was so long-winded, I tried to explain it as best as I possibly could. My mom has been a stylist for almost 30 years, and has taught numerous classes on hair styling techniques for thermal styling.... so I have learned a lot from her. Hope I could shed a little light on my perspective.