  The temporal and spectral properties of ultraluminous X-ray sources (ULXs,
L>2x10^39 ergs/s) and bright X-ray sources (L>3x10^38 ergs/s) are examined and
compared in two extremely different host environments: the old elliptical
galaxy NGC 1399 and the young, starforming Antennae galaxies (NGC 4038/4039).
ULXs in NGC 1399 show little variability on either long or short time scales.
Only 1 of 8 ULXs and 10 of 63 bright sources in NGC 1399 are variable at a
confidence level of 90%. On long timescales, the NGC 1399 sources are steadier
than most Galactic black hole X-ray binaries, but similar to GRS 1915+105. The
outburst duration of the NGC 1399 sources is about 20 yrs, again, similar to
that of GRS 1915+105. The bright X-ray sources in NGC 1399 may be black hole
X-ray binaries with giant star companions similar to GRS 1915+105. The
brightest ULX (PSX-1) in NGC 1399 is coincident with a globular cluster, shows
a hard spectrum with a photon index around 1.5, and has a nearly constant
luminosity around 5x10^39 erg/s. It may be an intermediate-mass black hole
(IMBH) in a hard spectral state. In contrast to NGC 1399, the ULXs in the
Antennae are all variable and a large fraction of the bright sources (9 of 15)
are also variable. The variability and luminosity of ULXs in the Antennae
suggest they are black hole high mass X-ray binaries accreting via Roche-lobe
overflow. A flare with a duration of about 5 ks is found from Antennae X-42.
The most luminous ULX, X-16, with a very hard spectrum (Gamma=1.0~1.3) and a
luminosity which varies by a factor of 10, could be an IMBH candidate.
