I just realised I need to update my world spawner lists. I've created a list of all the different spawner types that appear in EA worlds, by opening each world in CAW. This helped me to place a balanced number of seed spawners in my previous world. Recently, however, I found the exact difference in each seed spawner: It's the chance for a particular type of seed, and the quality range.
Personally I like to collect seeds from all the special seed spawners (I find them with CAW) to ensure that they spawn as many possible special seeds. I have to add a few worlds to my list, but from what I have, Sunset Valley has an abundunce of crap seeds. Twinbrook on the other hand, has all medium to higher quality spawners.