I just ordered the bc max low kit and was also comparing to D2. Went with BC for the following reasons
1) Customer Support= No comparison here. You can order replacement parts online and the people who pick up the phone are nice and knowledgeable about what they sell
2)Previously owned BC max low for a 2013 Camry that was awesome, basically lower the entire bumper on the ground if I wanted to
3) Only mfr who offers normal and max low as well as ease of customizing (spring rates, spring lengths etc)
4) Fixed sway bar endlinks for BC adjustable for D2. Like just slapping the correct endlink on without having to geek out on if I adjusted the endlink correctly
5) BC uses brake line mount welded to lower mount, D2 uses a cheap ass zip tie thing to mount brake line
D2 is 130 dollars cheaper and readily available however without the 5 week lead time of BC, but figured long term I would have more adjustment with BC and ordering coilovers is something I only want to do once
not sure if there is a 100% definitive answer to your query. Over the years ive messed around with different spring rates and found going higher in the rear made my cars too twitchy and overly precise. I like understeer since it is more forgiving and tends to wander less on the highways. Also i figure since the majority of weight is more in the front a heavier spring will help compensate. I am also using a progress rear bar that has balanced the car out nicely. Since the rear and front rates are so close the difference im sure is negligable anyways