I put a lot of thought into this before deciding on a skoolie. For nostalgic reasons, the idea of a skoolie has always appealed to me. On the other hand, both vans and tiny houses have points in their favor. In the end, it came down to a few simple rationalizations.
Vans are small
Yes, I'm looking for something "small," but vans push it a little farther than I'm willing to go at the moment. I'm not looking for something to live out of for a while; this is going to be my permanent home. As such, I want a few creature comforts... like the ability to comfortably stand inside and a traditional shower.
A THOW isn't self reliant
In my current position, I'm struggling to get the money together to build a bus... let alone buy a trailer, build a house, and then buy a vehicle capable of pulling it all. A bus has the advantage of being self-contained.
The price is right
A decent condition used school bus is cheaper than both a van and a trailer to build a tiny house on. Additionally, being entirely enclosed allows me to build in stages while still living in the bus (once a certain point is reached in the build process).
Mobility
Vans are more mobile than a bus, but a bus is more mobile than a THOW. Given the other considerations, a bus seems like the right compromise for me.