Each cutting edge must be ground to the same angle to ensure that they are doing an equal amount of work, so they must be precisely the same size and shape. Cheap cutters have very thin carbide tips that leave little scope for re-sharpening. As well as giving longer life, a thicker tip will give much more support to the edge and will stay sharp longer.
Check that the brazing on these tips is clean and that there are no gaps or voids, or the tip may detach itself under load. Poor quality brazing will also affect the ease of chip clearance. The shank should be smooth and perfectly true, with a nicely chamfered end and most importantly, dead on size.
If you have any doubts, check the diameter with a micrometer, as an undersized one will very quickly damage the collet. Then if possible, put the cutter into the router and feel how it spins, it must run free of any vibration.
If you are buying a bearing guided cutter, check that the mounting pin is strong enough to support the bearing and that the holding screw tightens sufficiently to get a good hold, but the bearing itself should have minimal play whilst still turning freely.