There isn't really actual proof that Rainbow guarantees better cards, so those players in the other thread may or may not have gotten Lady Luck smiling at them...
I don't know. I got Jackie in Rainbow before she went Cr, X-0DUS and Spyke in New Blood, unfortunately nothing special that I remember in Elite. I recommend Elite for when cards turn Cr, this way you get a higher chance of obtaining them.
Once I get a lot of credits together, I always buy a bunch of packs at once, all Classic with the highest clans (oftentimes including Vortex, Berzerk, Jungo, and Piranas lately). My last pack is always a Rainbow pack. The Rainbow pack has *always* given me something good.
There are a few ways to decide which to pick. Based on your estimation of personal luck:
Compare minimum value of any cards possible (Junga wins last time I checked, their lowest was about 300, while most others have a 150).
Compare the maximum value of any cards possible (Vortex holds this one till they get a Cr basically).
Compare the average value of the commons in the pack (this is for the pessimists who aren't QUITE as bleak as the first option is for).
Compare average value of the rare cards in the clan.
Compare the weighted average of all the cards (you have to assume some distribution for how rare/uncommon/common arrive. This one is the ideal for those who go through a LOT of packs, since in the end the average is what happens)
Newblood is typically touted as the best pick without the clarification that you should save your credits until a good release and blow them all on the very day of the release, selling the cards off immediately if you get from the new 4, and holding on to the cards which have already been out a while to ride the inflation in price once they leave Newblood.