I've 50 credits right now. Which do you think will earn me the most clintz? Spending 40 on both Solo packs and completing all of them to earn the Clintz in their rewards, or spending 50 to get 4000 Clintz immediately? The Solo choice has more chances of me getting more Clintz per Credit value, but it takes time and I may not complete some fights. The Clintz in the packs will get me me less Clintz per Credits value, but it's instant and fast. Any suggestions?
Basically, it's going to come down to how lucky you feel and how patient you are.
Most reliable, as you said, is to just buy the 4000 clintz right now.
In the middle would be the Solo missions. Assuming you finish all the missions (which can be tricky), you get 85 tickets for each "pack" of Solos. Depending on how well you do with the slots, you could end up very rich. It's my recollection that I pulled in about 2500 clintz or so for each pack of Solo missions.
Most risky would be buying 2 New Blood packs when the next set of characters come out on November 30. At worst, you'll get about 800 clintz worth of cards from each pack; at best, it could be anywhere from 30k to upwards of 60k. You do have to wait a week, though, and again, no guarantees.
Good luck with whatever you choose, and please let us know what you decide and how it turns out.