The 1500 came with two-track erase, record and play heads, plus the extra head was quarter track. The 1506 came with quarter track erase, record and play heads, plus the extra head was half-track. Thus if you are interested in playback only, one's as good as the other. This is true whether you intended to use the built-in electronics (crappy sound) or external electronics.
Then there was the 1700, which was quarter track all the way, but with auto-reverse (the extra head was quarter track for reverse play.)
And the 1800, which has collector value out of all proportion to its inherent value, so it need not concern us here.
And the 1520, which was like the 1500 above, plus it has balanced inputs and outputs, a built-in oscillator, and switchable IEC/NAB EQ. So on the surface it looks like a good candidate for Tape Project tapes. However, may I remind you that the sound quality of the stock electronics is really really crappy.
1500, 1506 and 1520 all make good transports to use with external electronics.