Yes when working to a tight budget the case is way down on the list for most people.
Looks are the other big issue, what you might like someone else may hate, so what you think looks good is a big issue.
personally I don't like the 310 front bezel, I'd prefer it with the mesh front of the 335
http://www.coolermaster.com/product.php?product_id=5239 although I much prefer the extra width and hard drive bays of the 310.
I really like the UPS100 in black but for some unknown reason they sell it in Asia only
http://www.coolermaster.com/product....tegory_id=3588
"Popularity" is generally just down to how many have been sold, not how good a product is.