There's extra information here (AN607, 253kB) about
oscillator start-up times. No specific mention is made of
differences between the three you mention, although if the
1024 Tosc applies to you then the 12MHz option would
appear to start sooner. The lower limit seems to be set by
OST and POR, but I haven't digested all the information
yet

http://www.microchip.com/1000/suppdoc/appnote/category/listing/945/index.htm

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