I don't own the PB360, so hopefully someone who has used both will post here also. But from just looking at the 360, the #1 thing I love about the Sony over it is the touch screen. The Sony doesn't have any expansion slots, but it will hold about 1200 books (and really is there any reason you need to keep 20,000 with you and need expansion slots?). From the
Wiki eReader Matrix here, they seem pretty comparable in size and weight, looks like a bit longer battery life on the Sony and it has the newer/better e-ink screen.
If you Google "Sony PRS-350 vs Pocketbook 360" you'll find more info and comparisons that might help in your decision. I can say the Sony looks and feels like a quality piece of hardware when you hold it, no plastic at all.
I can't say enough good things about it so far.
Edit: After looking at a few videos of the operations of the BP360, I will say that the Sony turns pages MUCH quicker than the PB360. The hard shell case is nice, but I'd still run into the problem of needing a light at times, so it doesn't save me the cost of a cover (I opted to get the Sony cover with light).