@Wizardling I can't speak to the general design choice for an outward blowing fan in most consumer electronics blow outward. This won't prevent dust accumulation because what goes out must come in somewhere else. This is usually remedied with air filters of some kind.

I use an inward blower and it seems to be working great. My Pi3 generally runs about 20 degrees cooler with the fan. I also use a heatsink applied with arctic silver thermal adhesive so the cooling is quite efficient. I'm not concerned about dust buildup because I just use it for gaming sessions that last no more than a couple hours once or twice a week so I imagine to get enough dust buildup to cause a problem will take years.