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.