  How complex organic - and potentially prebiotic - molecules are formed in
regions of low- and high-mass star-formation remains a central question in
astrochemistry. In particular, with just a few sources studied in detail, it is
unclear what role environment plays in complex molecule formation. In this
light, a comparison of relative abundances of related species between sources
might be useful to explain observed differences. We seek to measure the
relative abundance between three important complex organic molecules, ethylene
glycol ((CH$_2$OH)$_2$), glycolaldehyde (CH$_2$OHCHO) and methyl formate
(HCOOCH$_3$), toward high-mass protostars and thereby provide additional
constraints on their formation pathways. We use IRAM 30-m single dish
observations of the three species toward two high-mass star-forming regions -
W51/e2 and G34.3+0.2 - and report a tentative detection of (CH2OH)2 toward both
sources. Assuming that (CH$_2$OH)$_2$, CH$_2$OHCHO and HCOOCH$_3$ spatially
coexist, relative abundance ratios, HCOOCH$_3$/(CH$_2$OH)$_2$, of 31 and 35 are
derived for G34.3+0.2 and W51/e2, respectively. CH$_2$OHCHO is not detected,
but the data provide lower limits to the HCOOCH$_3$/CH$_2$OHCHO abundance
ratios of $\ge$193 for G34.3+0.2 and $\ge$550 for W51/e2. A comparison of these
results to measurements from various sources in the literature indicates that
the source luminosities may be correlated with the HCOOCH$_3$/(CH$_2$OH)$_2$
and HCOOCH$_3$/CH$_2$OHCHO ratios. This apparent correlation may be a
consequence of the relative timescales each source spend at different
temperatures-ranges in their evolution. Furthermore, we obtain lower limits to
the ratio of (CH$_2$OH)$_2$/CH2OHCHO for G34.3+0.2 ($\ge$6) and W51/e2
($\ge$16). This result confirms that a high (CH$_2$OH)$_2$/CH$_2$OHCHO
abundance ratio is not a specific property of comets, as previously speculated.
