Picture Quality - DVD vs high quality downloads

On the face of it, a DVD is a much lower resolution format so should have lower picture quality than a HD Rip. However, my doubts come with two things
1. Higher resolution doesnt necessarily mean better picture quality
For example, a 8.0 megapixel point and shoot clicks photos far worse than a 5.0 mega pixel SLR due to the colours and the richness of the photo quality.
2. A dual layer DVD takes around 8GB of space, A HD format takes 25/50 GB space. So a 2 GB file is compressing a lot of data, so will it maintain the quality.

On the flip side, the DVD format will need to be stretched in a Full HD tv while the H.264 rip would not need to.

So has anyone checked the two and compared which is better. Take an original dvd (dual layer) and compare it with a HD rip on a HD ready and a Full HD screen and found differences?

i have found the HD rips to have better PQ quality when played , DVD looks good, but can't match the PQ of a HD rip.

But how do you know before you download that the PQ is going to be good? Is there any metric for the same? Or its just the reputation of the ripper thats the deciding factor?

There are few things to look out for like -

1. Known and famous rippers whose releases you can download eyes shut, as they are always going to be good.

2. Unknown or new rippers you can always download the sample file first to check out the quality

3. Look at the screen shots posted & post of other members who have downloaded the same and their comments.