I'd tend to say buy BOTH (or all three), but I won't. /ubbthreads/images/graemlins/grin.gif
I have all three of these. I EDC the micra and P4. I use the micra alot, much more than the P4. They're all nice IMO. All have stainless tools and lanyard ring.
It depends on whether you prefer scissors or pliers. The P4 (pliers) and S4 (scissors) have the same knife and screwdrivers(2 flat, 1 phillips). In addition the S4 has nail cleaner/file and removeable tweezers(SAK type). In place of these the P4 has awl and metal file. They're both well made, but the anodized handles will get scratched up. The tools don't lock but they snap into place well. They're also the same size and both are spring loaded.
The micra is basically the same as the S4, but the implements, including the scissors are larger/longer than the S4. It also has a bottle opener. Tweezers are OK and unremoveable. The tools neither lock nor snap into place. It has a semi-useable inch/mm ruler on the handles.
Leatherman tools tend to rust a little bit in humid environments. It hasn't affected operation. Higher carbon content stainless I guess.
The other mini multi tool I EDC is the Swiss-Tech Microtechnician plus. It has pliers, wire cutter, shear, large and small phillips, large and small flat drivers. It's all hardened steel, black oxide finish. Not stainless so it rusts a little, but it hasn't affected operation. The pliers fold and lock onto a keyring very well. It's a tough little tool. Smaller than the other three.