The aluminum ones tend to sweat more than the vinyl. May not be an issue in your dry climate.
Typically the vinyl frames tend to have better R value. That being said, I'm leery of the cheap ones and how long they will last.
Death before Decaf! "...where did the last hour go? Oh yeah, reading this thread..." JBSmith
"On the road of life, it's not where you go, but who's by your side that makes a difference!" Calvin & Hobbes