Welcome to MobileRead, kucing!
I haven't heard anyone saying that there's really
any difference in page turn speed, etc. between the two. They both use the same display panels, so there's no difference there, but Sony uses a more advanced driver module ... I have trouble believing that a more advanced module would give slower results ....
Now the Gen3 probably
is faster than the old model 500, but if you're talking about the 505 ... nope, I'm not buying that.
JSWolf has mentioned the difficulties with Letter/A4 Sized PDF files. So I won't go into that in detail, except to agree with him that if what you're primarily wanting to read is that sort of PDF, you stand a really good chance of not being happy with
any 6" device. PDFLRF will give you a manageable solution, without much processing, but you're stuck reading half-pages. If that works for what you need then it's a great solution, if not ... then not so much.