#1
When I play a blues progression  I IV V (G C D)  using 7th chords.....12 bars, I find myself  using the C pentatonic scale, sounds better to me. Why not the G major...G major pentatonic is the same key?
#2
G major and C minor are not the same, but blues isn't diatonic in the first place.

Typically, when you play over the IV chord, playing in the key of the IV (either dominant or minor) tends to sound pretty good.
#3
maflake 
Either "a blues progression  I IV V (G C D)...I find myself  using the C pentatonic scale" contains a typographical error or you are playing something else for the progression or C pentatonic... because they don't fit and should sound awful. The major blues are called for as dominant sevenths, but that is to allow for flexibility. Actual sevenths are sometimes used - more likely ninth chords and sometimes thirteenths are substituted (a lot easier to hear the blues sound with these chords). Be sure your Roman numerals follow the convention of upper case for major and lower case for minor, then check to see why what you think you are playing and what you are hearing must be different...

G ninth has G B D F A
C minor pent has C Eb F G Bb
C major pent has C D E G A

C ninth has C E G Bb D
C minor pent has C Eb F G Bb
C major pent has C D E G A

D ninth has D F# A C E
C minor pent has C Eb F G Bb
C major pent has C D E G A

Notice the clashes between those scale notes above and the chords.

G major is G A B C D E F#
G major pent is G A B D E

Yes, same key, but a G major scale with F# will not be good for dominant blues... notice that the G dominant 7 is a flat seven (b7), so it is F rather than the F# of G major.

Now there are ways to play a "C scale" with a dominant blues in G, but it needs to be something like the C Lydian Dominant (b7) where its inversion with tonic on G will fit with the G dominant chord. 
Quote by reverb66
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#4
All of the notes in C major pentatonic are also in the key of G major. The scales are just one note different (G A C D E vs G A B D E), but with G major pentatonic you emphasize the major sound a lot more because of the major third (B).

Also, maybe you find the sound of G major pentatonic over G major blues to be a bit "generic" and maybe the C major pentatonic just sounds different in a good way. I don't know. It has everything to do with your preferences. But both should technically work just fine (especially if you know which notes to avoid emphasizing over which chords).
Quote by AlanHB
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#5
Note nr. 2: Sometimes its helpful to play chromatically over a blues progression to figure out what notes work in different situations for different purposes.
#6
Here's a basic recipe for blues:

Over the G7 :  play G mixolydian - the flat 7th ( i.e. the F note) is a super important note, as is the major 3rd ( B note, which you will want to slide or bend into alot) - those are the "money" notes over that chord.  

Over the C7 : play C mixolydian ( which happens to  have the same notes as G dorian) - here you can get away with playing the G minor blues scale as well - accentuate the "B flat" note and the E note - those will will highlight the chord change and are your "money" notes. 

Over the D7 : this is the tricky part- D mixolydian will work, but just make sure you focus on accentuating the 5th, 3rd and flat 7th of D, and you'll be fine.  The 5th ( A note) is really a great note to hit to highlight that change. SRV does that all the time for example. 

The important part of playing blues is to switch scales for each chord and to try to land on or accentuate the money notes.  A listener should be able to tell where the progression is at just by hearing your solo by itself.  
Last edited by reverb66 at May 31, 2017,