- In general, round reels have a larger diameter spool than low profile
reels. This means that the line speed (in feet per second) will be faster
in relation to the spool speed (in RPMs). While this means that a round
reel's spool doesn't spin quite as fast on the cast, (and I guess in theory
the bearings should last longer), there's not much perceptible difference in
that direction.
The real difference is on the retrieve. The spool of the low profile reel must spin faster to retrieve a lure at the same rate, because of the smaller spool diameter. If the spool is "slower" in inches per revolution, then the gear ratio has to be higher to achieve the same effective retrieve ratio (inches of line per HANDLE revolution).
- All else being equal, there is one 'given' in gear drives, regarding the
strength of the drive. The larger the meshed area, the stronger the
coupling.
Round reels have room for larger gears, and because of factor 1 (above) they don't require as 'steep' a gear ratio. This works out to theoretically stronger and smoother gearing in the larger diameter reel. I say theoretically, because all else is rarely equal. Wider gears don't take up any more space (diameter-wise) so it's easy to achieve the same amount of meshed area with smaller gears that way.
Materials and the precision of the mesh, as well as the angles the gears are cut on can also alter the dynamics of the gear drive.
In other words, while the diameter limitation does complicate the engineer's ability to design the same degree of smoothness and strength into the gear train of a low profile reel's gearing, it's nothing that can't be overcome by throwing money at it.
So if you're buying a low end reel, you're more likely to get functionally better gears in a round reel, but once you get into the pricier stuff, the difference probably doesn't amount to squat. Then again, it might not amount to squat in the low end reel either, because just having the room to make bigger gears doesn't mean the manufacturer actually took advantage of that room.
In the end, you just need to bear in mind that a low profile reel usually needs a slightly higher gear ratio to achieve the same retrieve rate.