I have done both 669 and 690 emulsion transfers. I do the 690 transfers dry, peeling at about 30 seconds then placing onto paper. The other trick is to peel apart away from direct light (I do the peel upside down). If light hits the 690 prior to getting to your paper, you can get an overall yellow cast to the image. If you get that yellow cast, then you can run water slowly over the image and very gently rub your finger across the yellow areas. That action can also be used to lighten areas slightly.
The main difference is that 690 is more vibrant and slightly better contrast than 669. So instead of muted emulsion transfers, you get much more saturated colours. Overall I like the 690 transfers more than the 669 transfers.
After trying many papers I have stuck with one for all my Polaroid manipulations. That is Fabriano acquarello Artistico Extra White Grana Satinata Hot Pressed Watercolour. This paper comes in blocks, though it is easy enough to cut to size with an Xacto. I usually get the 23x30.5cm 300g/m2, which gives 20 sheets, or roughly 80 Polaroid sized pieces of paper.
Ciao!
Gordon Moat Photography