First, a quick lesson: pex has a memory (i.e., it likes to go back to its original shape). When you crimp, you are going against it's normal desire to stay full size. In order to insert a fitting, it has to be smaller, and thus creates some restriction so you have a chance of pushing it into the tubing. Then, you go against its normal desire by crimping it to hold that fitting in place. This works, but if you don't get it crimped properly, can be a problem with leaking. If you instead, use an expander tool to make the opening bigger, you can then insert a larger fitting, and then, when the tubing returns to its normal, preferred size, it crimps itself. Now, to make this stronger still, before you expand it, you add a ring of pex on the outside of the tubing where the fitting will go and expand both of them. Then, shortly after you insert the (larger ID and OD) fitting, it's solid, and a leakproof joint.
Also, pex is manufacturered in three basic types based on the manufacturing method, called type -A, -B, and -C. The only version that can be restored to normal operation if you kink it is type -A. If you kink -B or -C when you are pulling it, you are supposed to cut out the kink and insert a fitting. The practical differences, all things being equal, are small, but -A is the most flexible since it has the most complete cross-linking - this means it can be bent into the smallest bend without kinking in the first place. Uphonor makes -A stuff, and is one of the oldest manufacturers of the stuff. -B and -C were developed to try to make the process faster and cheaper, and, generally, it does (but the difference isn't huge). But, -B and -C do not have the same level of cross-linking as type -A (and thus the flexibility and strength). When there have been problems with pex, it has been with types -B and -C. The other area that has been a problem with some is the fittings, where the water eroded the material and they became brittle and broke. This is worse when there is a restriction, since the water flows faster through that (short) restriction (acting like a venturi).
A crimping tool is cheaper and easier to use, especially in cramped places since you approach the tubing from the side. The expansion system requires you to access the end. The tools are also more expensive. The powered ones even more. The manual expander tools work fine, but require more strength to operate. Often, though, you can buy one, and sell it when done, and not be out a lot of money if you don't need to have one sitting around. With a crimping tool, if you don't get it right, it can leak. WIth the expansion system, assuming you insert the fitting all the way, your success rate is higher and it doesn't require the (typical) go/no-go gauge to test the thing is actually crimped properly.
