The second method, and the one which I prefer and use ALOT in my blues and rock playing, is to use the two different scales over the two different chords.
For the A7 type chord (I) I play the Major Blues Scale. When the D7 type (IV) chord comes, I use the Minor Pentatonic. Making this change of scales at the appropiate time really defines the chords that are happenning and reinforces the Blues theme of Major sounds to minor sounds and back and forth.
LOOP this progression FOREVER and practice making the scale change on the first beat of each new bar.