I am comparing 2 arduino mega's.
The DUE is not a Mega.
Does it mean that if i set a pin to high that it's voltage will be 3.3 volt ??
Either that or 567. 3.3 seems a lot more likely, though.
and is that enough for normal shields ?
Yes, if the shield is a 3.3V shield. There are shields that need 3,3V and there are shields that need 5.0 volts.
I am planning to use a mosfet shield to drive lots of leds
Which MOSFET? Does it trigger at 3.3V or 5V?
There are very few shields that connect to all the pins of a Mega or DUE.