G-Series vs. SX-Superzooms??

Pretty much, particularly in lower light situations -- and especially so for the G15 with its faster lens.

In many situations the IQ differences will be no big deal (e.g., "vacation" shooting in daylight condx).  But the G15 and others of the series also provide more flexible shooting controls and other features (e.g., built-in neutral density [ND] filter).

--
Phil