typically a speaker with a higher impedance is more desired than one with a low impedance, especially if you have a tube amp - which will sound like it has tighter bass if your speaker has a 12 OHM load instead of a 4 OHM load for example.
A tube amp does not change power output when speaker impedance changes like a solid state amp does, when you lower impedance into a solid state amp they put out more power but a tube amp usually puts out about the same amount of power when the impedance changes.
Usually speaker impedance is more important when you have a tube amp.
This is a very rough answer and only a guideline as there are exceptions - and certainly better answers - I am sure.
A tube amp does not change power output when speaker impedance changes like a solid state amp does, when you lower impedance into a solid state amp they put out more power but a tube amp usually puts out about the same amount of power when the impedance changes.
Usually speaker impedance is more important when you have a tube amp.
This is a very rough answer and only a guideline as there are exceptions - and certainly better answers - I am sure.