Hi David,
1. You can either compare noise density or RMS noise. I suggest RMS noise since for the same noise density, different filter design may cause the final RMS noise a little different.
2. RMS noise = nosie density * sqrt(filter gain * BW), for the one pole filter, filter gain = 1.57; For two pole filter, filter gain = 1.44.
3. You can find ADXL345 noise density on page 33 of datasheet.
4. ADXL103 has lower noise than ADXL345. It is used widely in industrial market for tilt sensing and platform stabilization.
BR
Neil