Should I buy a PVC or aluminum roller?
How big of a difference is there
  between them?


The difference is this big...

http://www.flickr.com/photos/bobmarley753/249120602/

Seriously though, after some research and asking friends, many who have aluminum rollers prefer them over PVC rollers

The unasked question:


  Which one would you buy?


Neither!

Some of my friends have rollers that are cylindrical like the two you have linked to. When asked about their rollers, their biggest complaint is how often they end up riding off the side of their rollers. One even has gotten in trouble with his wife after riding off the edge of his rollers leaving burn marks on the floor. oops! With cylindrical rollers, more concentration is needed to stay on, which can be a good thing if that is something you are looking to improve in your form and concentration. 

If I were to buy another set of rollers, I would most definitely purchase another set of parabolic rollers.


I love my parabolic rollers! They were really easy to learn on and I have yet to ride off the edge of them. Not having to worry as much about riding off the edge is really big for me as it allows me to ride longer, feel more comfortable shifting my hands to shift and readjust, and allows me to watch an occasional movie or two while riding. : ) If you are looking at getting a set of rollers, I would highly suggest adding parabolic rollers to your consideration list.

Google search for parabolic rollers to get you rolling ...chuckle

My thoughts about having ridden on plastic rollers:


Never had my rollers statically shock me.
Haven't seen any major wear on the rollers that makes me worried that they won't last for years on end.
Love how quite the rollers are especially compared to my CycleOps Fluid 2 Bike Trainer.
Never needed to use them outside.
Has been a very smooth ride for me and much smoother than my fluid trainer.
Resistance? I love that my rollers have a variable resistance module so I can change the resistance depending on what kind of workout I am going for.