That statement catches my eye for one reason, tri bike. Rollers + Tri bike = death trap for most types of rollers.

With sufficient practice and a good set of larger diameter (4.5s) drum rollers you can ride a Tri bike on them, but in my experience it is not pleasurable. There are of course exceptions to this, TruTrainers and the E-Motions are both very nice, but both are expensive (~$800) and you are about as likely to bring a pile of cinderblocks to the park as you are to bring the E-motion anywhere. It's big, it's heavy, it doesn't fold, it's great as long as it's great right where it is.

As for PVC verus aluminum, I have not heard anything about the aluminum adversely affecting wheels, riding rollers induces far less wear on your tires and I would suspect less wear on your bike in general as the stress is more akin to actual road riding rather than a bunch of lateral torque like when stuck in a normal trainer.

In my limited experience with rollers however, it's all about the inertia. Large diameter aluminum rollers have more mass and have more surface area for your wheel, as a result the ride typically feels more like the road, including increased coasting when you stop pedaling. Lower diameter rollers (3.0s or 2.5s) offer far more resistance than larger diameter rollers, they give a better workout without requiring a resistance unit but at the expensive of some of the feel of the larger drums. I've never ridden full PVC, but I know that poly capped aluminum rollers are offered because they are cheaper than full aluminum but have less inertia because the PVC is less dense. I may be wrong, but I would guess that the real advantage of PVC rollers is price, not performance at all.

I guess to summarize, you need to know what you are looking for before you can figure out what is best. If you think you want portability, the E-motions are out, get a set of folding rollers. If you have tons of cash to blow get TruTrainers. If you think you are going to ride a Tri bike on them, get the E-motions or 4.5 full aluminum drums but you will probably end up needing to get a resistance unit.

I really wanted E-motions, but honestly did not want to spend that type of money. I ended up getting folding Krietler 3.0s that offer decent resistance without an extra resistance unit. I then built my own free-motion rollers. Search the forums or online for DIY free motion rollers and you'll see what I mean. I built my own bumpers to keep myself from rolling off the side and dying (or putting another giant tire mark on the carpet) and a free motion frame that I can set the rollers into. When I travel, I pull the normal rollers out of the free motion assembly and they are light and portable. The overall ride quality is still probably not as nice as the E-motions or TruTrainers but I saved about $500 in the process.

Edit: I based my free motion setup on this dude's: http://www.youtube.com/watch?v=StcY7bG1xzs