I would argue the synergies between cofounders to be the single most valuable entity in a partnership. You may have complementary skills but if you don't come together and are able to do incredible things that are spurred on by eachother. Spark is key - this linear trait argument isn't correct at all. Never compromise without spark.

You don't know by chatting with someone - best to work on something non trivial together and figure out if you get on.