I'd probably go for the sx610, although both say they can shoot well in lower light conditions,
it has more optical zoom and fast shooting mode. might come in handy?
besides older model, cheaper?
Newer doesn't always mean better
Shop around you may find other cameras with similar specs or better for less.
They both have the same size sensor (affects low-light performance) of 1/2.3". Both have the same number of effective pixels - 20.2 Megapixel. Aperture is basically the same f/3.8 - f/6.9 vs f/3.6 - f/7.0.
Canon PowerShot SX 610 HS
Focal length: 4.5-81 mm (35mm equivalent: 25-450mm)
Optical Zoom 18X (ignore digital zoom as optical zoom is far superior to digital zoom)
Dimensions: 105.3 x 61.0 x 26.7 mm
Canon IXUS 285 HS
Focal length: 4.5-54 mm (35mm equivalent: 25-300mm)
Optical Zoom 12X (ignore digital zoom as optical zoom is far superior to digital zoom)
Dimensions: 99.6 x 58.0 x 22.8 mm
The Canon PowerShot SX610 HS is larger and has a longer optical zoom. Both will perform about the same in low-light conditions and have the same shutter speeds.