  We present the results of a radio and X-ray study of the Galactic microquasar
GRS 1758-258, using unpublished archival data and new observations. We focus in
particular on the 2000-2002 state transitions, and on its more quiet behaviour
in 2008-2009. Our spectral and timing analysis of the XMM-Newton data shows
that the source was in the canonical intermediate, soft and hard states in 2000
September 19, 2001 March 22 and 2002 September 28, respectively. We estimate
the disk size, luminosity and temperature, which are consistent with a black
hole mass ~10 M_{sun}. There is much overlap between the range of total X-ray
luminosities (on average ~0.02 L_{Edd}) in the hard and soft states, and
probably between the corresponding mass accretion rates; in fact, the hard
state is often more luminous. The extended radio lobes seen in 1992 and 1997
are still present in 2008-2009. The 5-GHz radio core flux density has shown
variability between ~0.1-0.5 mJy over the last two decades. This firmly places
GRS 1758-258 in the radio-quiet sequence of Galactic black holes, in the
radio/X-ray plane. We note that this dichotomy is similar to the dichotomy
between the radio/X-ray sequences of Seyfert and radio galaxies. We propose
that the different radio efficiency of the two sequences is due to relativistic
electron/positron jets in radio-loud black holes, and sub-relativistic,
thermally dominated outflows in radio-quiet sources.
