I'd use the strong stuff. Doesn't cost much more than the weak stuff, and will serve your needs much better.
OK, you can smack me in the face for that smart-aleck remark, but I couldn't resist it.
What concrete you use would depend on a few things, such as availability, how you intend to load the piers, etc. If, by piers, you really mean concrete footing pedestals to support point loads from steel or timber columns, I don't think you'd need much more than a 2500 psi mix. If your piers are actually columns, you could go with 3500 psi. If you're batching the stuff yourself, I'd go with Quikrete 5000 or an equal. It develops a decent higher early strength, and you can always sweeten it up with some Portland cement to make it even stronger. Don't go overboard, or you'll be asking for cracking problems. And some tie bars (hoops) to contain the longitudinal compressive reinforcement in your columns wouldn't hurt, either. Even if it's just pedestals you're pouring, some rebar (No. 4s at 10", both directions) is in order to tie things together.
The stouter the mix, the sooner you can safely apply loads to it, without damaging anything. I'd wait at least 4 days for a stronger mix to develop reasonable strength, and 7 days would be better. For weak, typical footing concrete, I'd wait 2 weeks before any heavy loads are applied. A quick shot of curing compound after forms are stripped will add significantly to the strength-gain curve.