What's the main differences between a TSSOP and a SOIC and when would you use one over the other?
The SOIC is more than 50% longer than the TSSOP. (4.9mm vs. 3.0mm) and only a bit wider. That may not seem like a lot to you, but on a crowded board it might make a difference. 

The SOIC is taller (1.75mm vs. 1.2mm) which is enough to make a difference in a thin product. 

The lead pitch is much closer (almost half) on the TSSOP- 0.65mm vs. 1.27mm, so for crude manufacturing processes the SOIC might well be preferred. If you think they are the same to hand solder- give it a try, unless you are quite skilled you'll see quite a difference.