If you are a heavy clencher, you put extreme stress on each tooth.  Most
clenchers will also attempt some side to side movement while they are
clenched.  The teeth are so tight together that they cannot actually slide
the teeth side to side, but they do bend the teeth (ever so slightly) --  
think of a tree bending in the wind.

If you have a gold crown placed over a tooth which already has a big filling
in it, the outer surface of the tooth will have to be reduced.  At the
margin of the crown (near the gum-line) this is only 0.5 mm, but half-way up
the tooth this can easily be 3-4 mm (due to the convexity of teeth).  Now
the remaining tooth structure inside the crown is very, very thin.  As you
try to force your jaw from side to side while clenching, the tooth tries to
flex and spread that force out.  The gold is very rigid and cannot flex.
The portion of the tooth root embedded in bone cannot flex, so all the
flexure has to occur in the 3-5 mm between the bone and the gold.  If the
tooth is thin under the crown, the entire tooth snaps off at the gum-line.

If you have a CEREC onlay done the sides of the tooth are NOT cut.  The
tooth retains whatever thickness the walls have between the filing and the
outer enamel surface.  This ceramic has a modulus of elasticity much closer
to tooth than gold.  As you clench forcibly and attempt to slide the jaw
around, the flexing is spread from the edge of the bone to the tip of the
tooth.  Much less likely to snap the entire tooth off.  In five years of
making CEREC restorations, I have not had one tooth break off that way.
Prior to CEREC, we had 1-5 teeth break off at the gum-line every year in our
practice.  If you have a CEREC crown made, you will still have the sides
thinned out.  I suggest not cutting the outer walls of any tooth unless
there is decay or discoloration there, or unless you are replacing an
existing crown.  Sometimes, due to prior damage you have to shorten the
entire tooth to restore it with a CEREC, but you can almost always preserve
the half of the tooth walls closest to the gum tissues.

When gold crowns fail, (in heavy clenchers) the tooth breaks off.  When
CEREC onlays fail, a small piece of porcelain chips off.  The chipped CEREC
onlay can be smoothed over, patched, or remade.  The broken gold crown/tooth
can sometimes be repaired with a difficult reverse core procedure, often
will require root canal therapy now; as well as posts down into the roots.
It might even need gum surgery to be able to fabricate a new crown.  At
best, the broken tooth will function for a few more years before it fails in
a catastrophic manner.  Most of the teeth which break off at the gum-line
after a crown cannot be saved and get extracted.

If you are not a heavy clencher, it does not matter what material is chosen,
as it will hold up fine.

A molar restored with CEREC does extremely well, so long as the porcelain is
at least 3 mm thick in the biting surface.  The average gold crown is 2 mm
thick in this region.  I do CEREC all the time on molars.  No big deal.  I
check every patient for anterior guidance when them come into my office the
first time.  Less than 20% have anterior guidance.  Virtually everyone
functions on some level of posterior guidance.

Wearing an NTI will improve your sleep, reduce your headaches and stiff
necks, reduce the number of times your dental work will need to be re-done,
protect your teeth from damage during sleep, and frequently never need
adjustment.  A horse-shoe shaped splint will do this 40% of the time.