You've summed it up very well. The ThinkPad Tablet 2 has perhaps the best battery life of all among the pure slate Atom tablets.
Be warned that the base model ThinkPad 2 and Dell Latitude 10 don't have a Wacom digitizer and pen. If you want that feature, make sure it's on the model you choose. In the US, Lenovo mostly lists the one with the pen, but some stores sell a version without that feature.
Staples has the HP Envy x2 on sale for a crazy $599 right now! You get the keyboard dock with secondary battery for that price. I'd jump on that if you don't need the pen feature. It works well as a standalone tablet too, and it's only a few ounces heavier than the lighter Win 8 Atom tablets.