I've trained in both Muay Thai and Kickboxing, well, the modern Kickboxing that we see these days like K-1.
The difference between AKB and MT can be seen in my sticky but to make it a bit simpler.
MT basically has 4 types of punches; swing, straight, jab and uppercut. There is very little dedication put into these techniques as most modern day MT boxer would rather be able to kick hard enough to break bones (idiots).
In AKB, the emphasis might be on kicking, but in close range, AKB becomes boxing with bob, weave, slip, and many evasive movements. There is almost the same level of dedication to your fists as there is in boxing when you're training AKB for it is essential. Let's just say, American Kickboxers can punch just as hard, hell, even harder than boxers sometimes.
What are their difference? AKB has better punching techniques and wider range of kicking techniques. MT has simpler techniques but more powerful elbow and knee techniques and wider range of them. AKB is faster paced but can create many opportunity for retaliation, whereas MT is slower but there is less room for mistake.
AKB is a solid decision. It's just as good as MT, and you'll be having fun learning boxing as well as some karate kicks. Trust me, 8 years of doing the same kick tends to be a bit boring.