SB10 (.459 bore) vesus SB12 (.464 bore)
Trumpets have what is called cylindrical bore, which is when the bore diameter is consistent throughout the instrument until you get towards the bell section when the flare begins.
Cylindrical bores give a well projected and direct sound. Typically, a medium bore (the SB10) may be a little easier to play from an air standpoint, have a slightly sweeter sound, and can benefit younger players. A larger bore (the SB12) may have a broader sound, have less resistance through range and volume, and benefit lead type players.
One is NOT a beginning horn while the other a professional, as many professionals play on medium size bores.
A sports analogy would be size and weight of a baseball bat… One player may like a smaller lighter bat to increase bat speed, while another may prefer a larger heavier bat to increase power.