My personal opinion is that watching them together is pointless. The OT is so significantly better, than including the PT in with the OT just adds a bunch of needless crap. I'd recommend watching each trilogy separately, separated by at least a week. That way, the PT won't damage one's enjoyment of the OT, and the PT can be enjoyed in its own separate way - as a bunch of kinda cool eye candy.

Watching them together is like eating a dinner that has both sushi and dog food. The dog food is likely to make it a little hard to enjoy the sushi.