I have not used or seen a YS640 in action; however, I did research pellet versus electric.
While pellet are very much set it and forget it, there are two concerns (maybe three) that impacted my decision not to get one. Suspect this means nothing as there are obviously many satisfied users!
1. Pellet wood is expensive. Yes, there is much more controlled smoke through the entire process; however the smoke really only penetrates the food the first few hours. That said, for true enthusiasts pellets offer a smoke ring (which is optics only and has no impact on the taste).
2. The mechanical auger. I personally didn't want a mechanical device that I would have to worry about coming apart or breaking down over time. Also, I cannot recall, but the YS wasn't stainless and that was a requirement for me as I wanted the unit to be with me for a good period of time.
3. Price. The price of the YS versus a comparable electric was ~30-40% above what was available at Smokin It.
No worries - smoked food is good food. Suspect it would be great if you could weigh in once you get the smoker and let us know how its going...