I just got through replacing the plumbing in my house with PEX.
In the beginning, I tried the bolt cutter style crimper and the copper rings, but
I switched to the "cinch" style that uses this kind of ring:
...and this kind of crimper:
The cinch rings are a bit more expensive, and the cinch crimper is cheaper and smaller.
The cinch crimper grabs the ring on the 'cinch' on the side. Once you start squeezing, a ratchet engages. You can't remove the tool until the chinch is complete. You don't use the 'Go, No-Go' tester thingie.
I had a couple of leakers with the copper ring style, and NO leakers with the cinch set up. Also the cinch crimper is shorter and easier to manage in tight spaces, because you just need to grab onto the cinch on the side of the ring.
The guys at the hardware store told me that plumbers were telling him that the cinch style was less likely to loosen up with temperature cycling.
Lastly, if you need to remove a cinch ring (I did because of a plumbing error, not a leak), the cinch ring is easier to remove without damaging the PEX, than the copper ring.
-AC_Hacker