Hands down, this is probably the most poplar question I get. I would say it comes second only to "how to I determine which size is right for me?"
The trickiest part of this question is that "effective" is highly subjective and open to a world of interpretation. So I will do my best to answer this one based on 3 possible meanings of the word "effective".
2. Which corset is best suited for waist training?
As mentioned in point 1, a steel boned corset is perfectly fine for waist training. Let's not forget that waist training simply refers to the practice of wearing your corset for an extended period of time (either throughout the day, or even through the night) on a frequent, typically daily basis. So it isn't required that you start off with a waist trainer and in fact, I like to recommend a simple steel boned corset for those who are entirely new to wearing corsets. I personally find them easier to get accustomed to since they don't nip your waist in as much as a waist trainer will.
Therefore, either a steel boned or waist training corset are perfectly fine for the practice of waist training and the overall style (lots of trim, or simple classic style; overbust or underbust) is entirely up to you and whether or not you plan on wearing it as an over piece (this is where trim and other such details will really shine), or beneath your daily attire (this is where you may want to consider a style that does not have any additional trim and is simple/classic in design).
3. Are overbusts better for waist training than underbusts?
Technically speaking, neither is actually better when it comes to waist training or waist reduction, but I do like to suggest underbusts for daily/regular wear. Not because they will provide better "results" or because they are more "effective" but rather because they are far easier to conceal beneath your clothes. Overbusts are gorgeous and can easily be incorporated as part of your outfit, but can be an entirely different story if you are trying to conceal the overbust beneath a shirt or dress.
So there you have: 3 possible answers to the possible meaning of which corset is most effective.
Love of Corsets.
Wife, mother workaholic & entrepreneur. I do my best to bring you regular content via this blog, but as you can see from my archives, it doesn't always work out that way (see above, where I mention the whole "mother" thing).