I have a Blackburn exp and a cheap knoeck-off copy on my daily commuter. the knock off gets a lot more use inc heavy shopping but such is the design with fully triangulated stays that it works fine. The triangulation adds stiffness in all directions, check out the Blackburn so you understand this feature.
If you are heading way out of bikeshop country a tubular steel rack may be useful but the Al rod designs have proven themselves.
I like a rack to be as simple as it can. A rear light bracket is a big plus.
Adjustable height vertical stays are pointless ; Ive never had to adjust the height of my rack, get one that fits and has no height adjustment mechanism.
A solid top plate can pevent mud splatter but if you use fenders, the open style top has more flexible choice of attatchment points for bungie chords.