They are pretty much the same. Normally, there is a thin washer between the bearing and the bit to help keep debris out. The bearing post will have a shoulder that keeps the bearing from contacting the cutter. There are differences in bearing size depending on the bit. A 1/4" shank flush cutting bit with a smaller cutter will use a small bearing, 1/2" shank bits with larger cutters use a larger bearing. Smaller bits for thinner material. In addition to the normal bearings there are Euro bearings which have a square plastic shroud around the bearing. These are used so the bearing doesn't turn against your work and leave a mark. Flush trim bits come not only in various sizes but the bearings can be bottom or top mounted. If you are template routing, the template needs to be about 1/4" thick for the bearing to ride on. The real difference between an inexpensive bit and more expensive brand name is the quality and thickness of the carbide, not much difference in the other parts of the bits.