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Aluminum is almost always the best choice for sliding glass patio doors, and I'm telling you that because it holds up to everything you throw at it without warping. It's more resistant to temperature changes, it doesn't expand and contract like PVC does, so your door will keep sliding smoothly five years down the line. Aluminum is also more durable, it doesn't yellow in the sun, and even though it costs more upfront, it pays for itself in the long run. If your patio gets a lot of direct sun or there are pretty significant temperature swings between day and night, aluminum is definitely your friend.
PVC has its place, though. It's cheaper and works fine in mild, stable climates, but in patios with heavy sun exposure or in places where it gets really hot and really cold, you end up with problems. The glass stays the same either way (use tempered or safety glass, no question), but the frame is what makes the difference. With aluminum you also get less maintenance - you don't need to constantly clean joints or worry about seals degrading.
If your budget is really tight and your patio is protected (under an eave or without constant sun), PVC works. But if you want a door that holds up year after year with no headaches, aluminum is worth the extra investment.
The truth is that aluminum has its advantages, but it all depends on where you live and how much maintenance you're willing to do. I have an aluminum patio door at my place and it works fine, though it's not as miraculous as it sounds. Aluminum does hold up well to heat and humidity without warping, that's true, but the thermal expansion they mention is real: with sudden temperature swings you notice the door moves a bit, and sometimes it's hard to close it in winter.
PVC gets a bad reputation for expanding more, but it really depends on the quality and thickness of the profile. More robust PVC is pretty stable, especially if you don't live somewhere with extreme temperatures. Where PVC clearly loses out is impact resistance and longevity: it scratches more easily, yellows in the sun (though there are treated options for that) and if it gets damaged, it's more complicated to repair. Aluminum is more resistant and durable in that sense.
What I'd do: if your patio gets a lot of direct sun and you're planning to keep it for 15 or 20 years or more, go aluminum without a doubt. If you're looking for something cheaper for now and don't mind replacing it in a few years, PVC works perfectly fine. It also matters a lot who installs it: a door that's poorly fitted will fail no matter what material it is.
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