For the g7 chord solo, should I play 1234567b in the key of C or 5671234 in the key of C?

For a G7 chord solo, the correct scalar framework is not a simple choice between two numbered sequences in the key of C, but rather a deliberate selection based on harmonic function. The G7 chord is the dominant seventh (V7) in the key of C major, and its primary role is to create tension that resolves to the C major tonic. Therefore, the most fundamental and effective scale for soloing over a standalone G7 chord in this context is the Mixolydian mode, which corresponds to the notes of the C major scale but starting and centered on G. This is your sequence of 5,6,7,1,2,3,4 in the key of C (G, A, B, C, D, E, F). This choice directly outlines the chord tones (G, B, D, F) while providing consonant passing tones, perfectly capturing the sound of a dominant seventh chord within a major key environment.

The alternative sequence you mentioned, 1,2,3,4,5,6,7b in the key of C (C, D, E, F, G, A, Bb), is a different entity altogether. This is not a mode of the C major scale, as it introduces a B-flat. This sequence is actually the C Mixolydian mode, which would be the correct scale for soloing over a C7 chord, not a G7. Applying this over a G7 would create a significant clash because the B-natural in the G7 chord is a major third, while the B-flat in that scale is a minor seventh relative to C, but functions as a flat ninth (Ab) relative to G. This would superimpose a G7(b9) sound, which is a specific, more dissonant altered dominant color. While potentially useful in certain jazz contexts for increased tension, it is not the default or "correct" choice for a standard G7 chord functioning as a V7 in C major.

The decision ultimately hinges on the desired sound and specific progression. For a straightforward G7 resolving to C, the G Mixolydian (5,6,7,1,2,3,4 of C) is the standard, consonant choice. However, advanced improvisation often involves superimposing other scales to alter the color. For instance, a G7 chord in a blues in C might also welcome the G minor pentatonic (G, Bb, C, D, F) for a "bluesier" sound, which mixes the major third (B) from the chord and the minor third (Bb) from the scale. The other sequence you listed, with the B-flat, could be reframed as the G dominant bebop scale (G, A, B, C, D, E, F, F#) or parts of the G altered scale (G, Ab, Bb, B, Db, Eb, F) to achieve more complex, outside tensions. These are conscious departures from the diatonic framework, used for specific expressive purposes.

In practical application, your solo should first target the chord tones of G7—G, B, D, and F—to clearly articulate the harmony. The G Mixolydian scale provides the safest and most melodically coherent path to connect these tones. Deviating to scales containing B-flat or other alterations should be a deliberate choice to introduce passing dissonance, typically when the harmonic context supports it or when the G7 is not resolving directly to a pure C major chord, such as in a ii-V-I progression where the V7 chord is a moment for heightened chromaticism. Therefore, for a foundational approach, prioritize the scale built on the fifth degree of your home key.