Just a thought on the rings vs. compression fittings. I have used, and still use if necessary, Oetiker clamps. These are the small, eared clamps that require a specialized plier. The pliers are available for well under $40 and are designed to install the clamps. I bought mine here
. One advantage of the pliers is that the pliers have two clamping faces allowing for some very tight installations to be accomplished quite easily.
will give you some info on the clamps and why you might consider them as an effective, low cost alternative to the screw together compression fittings. A big plus is the ability of the clamp to 'give' a little when the PEX pipe and fitting are under freezing pressure and return to full functionality when temperatures return to above freezing.
Just a thought.