I have been hearing this question all my life, and have asked it myself many times. The most common answers are:
1) It depends on how you cast
2) It makes very little difference percentage wise for relatively good casters.
Here is a very good article exploring that subject for any who are interested. 8 tournament distance casters in Europe each cast the same 8 rods (5 wt manufacturer stamp )with the same brand line 4 times indoors. The tape measured results of the 256 casts were tabulated and analyzed.
Four rods were American manufacture, the others European. The ERN number they use is the "Effective Rod Number" determined by what weight line is required to bend the rod to 1/3 deflection by using weights on each rod, rather than relying on what the manufacturers stamp on the rod. All rods in the test were stamped 5wt by the manufacturers but the ERN's vary considerably.
To me, it was an interesting, objective article. Just thought I'd pass it on.