It's easier to clean a snarl out of the vertical bobbins, especially front loaders like my Pfaff. I had an Ellure and a snarl under the embroidery hoop was a pain. With the Pfaff, cut the snarl, move the hoop (has button), clean out underneath, move the hoop back and continue embroidering. (Pfaff 2170 embroidery unit is at the back of the machine, not the side) With the Ellure, it was cut the snarl, take the hoop off, clean out the bobbin and futz with it to get it in just so, then hope you could figure out where to continue embroidering.
I've been told the vertical bobbins have more oomph for heavier fabric. The high speed single stitch are side load.
The Singer featherweight that everyone seems to love is a side loader. Most of the older machines that others use are either front or side loaders. I had a Riccar, and did teach some fancy sewing on a Basic Babylock Mechanical.
The Pfaff has a light for the bobbin, and I can tell by the sound when the Juki is running out.