  The identification and study of the first galaxies remains one of the most
exciting topics in observational cosmology. The determination of the best
possible observing strategies is a very important choice in order to build up a
representative sample of spectroscopically confirmed sources at high-z (z>7),
beyond the limits of present-day observations. This paper is intended to
precisely adress the relative efficiency of lensing and blank fields in the
identification and study of galaxies at 6<z<12. The detection efficiency and
field-to-field variance are estimated from direct simulations of both blank and
lensing fields observations. The presence of a strong-lensing cluster along the
line of sight has a dramatic effect on the number of observed sources, with a
positive magnification bias in typical ground-based ``shallow'' surveys
(AB<~25.5). The positive magnification bias increases with the redshift of
sources and decreases with both depth of the survey and the size of the
surveyed area. The maximum efficiency is reached for lensing clusters at
z~0.1-0.3. Observing blank fields in shallow surveys is particularly
inefficient as compared to lensing fields if the UV LF for LBGs is strongly
evolving at z>~7. Also in this case, the number of z>8 sources expected at the
typical depth of JWST (AB~28-29) is much higher in lensing than in blank fields
(e.g. a factor of ~10 for AB<~28). Blank field surveys with a large field of
view are needed to prove the bright end of the LF at z>6-7, whereas lensing
clusters are particularly useful for exploring the mid to faint end of the LF.
