I highly recommend the 8 Card packs. First off most of the time, you'll make more Clintz just selling the unwanted cards than you would have if you'd bought the clintz, especially if you sell the 5 star. Almost all of them sell for at least 2000.
Secondly the 8 card packs seem to turn up more rares than do the regular packs. I bought 8, and got at least (on average) 1 rare per pack, and a few I got 2 rares. Also the Uncommon ratio seems to be a lot higher on these packs as well.
The downside is that if you're looking for a specific team then you're almost better off buying the 3 card packs, as you're more likely to get that team, and I have gotten rares from those packs, but far less often than I have from the 8 card.
Overall it's a better value as well. If you buy the 3 cards it costs you 60 credits to get 9 cards. With the 8 cards it's 50 credits for.. well 8.
And 5k Clintz won't buy you the same level of cards that you'd get in an 8 card pack. (Nor would it buy you most of the 5 star characters, much less 7 others)