I have the plastic G10 4GB upgraded. Pretty nice tablet, haven't had any real problems except the weird battery drain, and the battery dropping to 90% almost immediately after a full charge. I got it from Pandawill. Spend about $150 on the G10, a screen protector and a leather case. Not bad.
I got the plastic because the plastic ones actually have better wifi signals. It's because of the odd placement of the antenna on the aluminum ones, not to mention the interference that the aluminum causes. But that is not why I got plastic. I got plastic because it's lighter. And the plastic is actually VERY durable and strong.
And what you said at the end is incorrect. The plastic and aluminum upgraded 4GB Gpads are exactly the same. The only thing that is different is the shell. Updates that work on the aluminum gpad will work exactly the same on the plastic (as long as they're both 4GB models). You're getting it confused with the 2GB plastic ones. I don't think they made 2GB aluminum ones. They didn't add aluminum option until the 4GB. But they DO make new plastic 4GB that are identical to the aluminum 4GB.
Hope you enjoy your Gpad. Share with us your thoughts on it.