Hi,
Yes, it appears that all BC548s are not created equal.
Fairchild appear to be happy to rate theirs at 500mA (
"This device is designed for use as general purpose amplifiers and switches requiring collector currents to 300 mA." ), but TBH I'd rather believe ON-dig/Philips and the BC337 (or one of many other types) would be a far more appropriate choice. Certainly I wouldn't buy BC548s specifically for this application.
Marmitas, I doubt if 12 volts (versus 5 volts) makes any significant difference. With a 5 volt rail, bipolar (or logic level FETs) can have a significant advantage over Darlingtons which have ~750mV saturation voltage (i.e. ~15% "wasted" voltage, or 30% potential power), but there's little difference from 12 volts, particularly if the base current is coming via a 7805 regulator from 12 volts anyway.
The amount of heat (power dissipation) transferred by conduction via the IC pins and convection from the IC package should not be under-estimated (if you think about it, that's almost the only way for the heat to escape), so where power dissipation is the limiting factor (as is the case here) then "piggy-backing" ICs is unlikely to help much. I don't know the physical difference between the two devices in the specification linked in #10 by Rick100, but the higher rated device may have a "heat speader" (e.g. a small lump of copper) inside tha package to help heat to escape, so a small heat sink "flag" glued to the package could be all that's needed.
IMHO probably any of the above solutions will work well enough, it really depends what components you have easily or cheaply available and how hard you might later "push" the design (to higher current LEDs, PCB enclosed in a box, etc.).
Cheers, Alan.