I find that reeds are a pain - firstly though, stick with one mouthpiece (I'd go for the bigger one, the 5C), then work on finding the reed to match. If 2's and 3's don't work, try a 2.5?
Reeds don't tend to 'go off' if they're new and dry, there's no sell-by date. They might go a bit green and smelly if you leave them for a while having played on them! Sufferers of Gear Acquisition Syndrome my want to purchase a humidity-controlled reed-saver...
I do find that on gigs I usually get through a reed fairly quickly, because I play harder - after this they still work, but don't have the same response. I can continue to use them to practice, in rehearsal, teaching etc. This would suggest that if your reeds are a little too soft for what you're doing try a harder one. Or try something like a Fibracell or Hahn synthetic reed that basically keep going until you split them.
If anyone else has any tips about what I'm doing wrong I'd be pleased to hear them...
Good luck,
Nick