If you've only got 28, just buy a NB pack and do tourneys for clintz. Never buy clintz using credits imho.
If it's 280, i'd say get some more and go for titanium or splurge on NB packs on new release days.
New blood packs.For clintz its better to play Daily Tournaments in order to receive decent amounts of clintz + more Credits(It is 1 Credit per DT).
I would not suggest New Blood. If you follow the link provided you will see that by getting the pack that is recommended you have a higher clint to credit ratio. I would like to note this worked for me I have pulled a kolor, Karrion, ongh, sylth, 2 chills, 2 raeths, spyke, Scubb and many other good cards. I have yet to get the most valuable piranahs though >.>