I've met Mary Jo in person and she is women of short stature, her lap is quite small, lapability depends on many factors, I'm 6.1 280 and the SP3 is lapable for me. I tested the SB and it work well in my lap.
I also have been able to use the SP3 on my lap (6'2" 250). SB seems better to me in terms of lapability. The wobble is a concern. I will not be able to know if the wobble is a deal breaker until I travel via train or plane next week. So far The device is a keeper. Will be selling my SP3 with docking station and case next week if all goes well.