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Some javascript applications can only be run in 64-bit mode, and even though it isn't my business; why do you want to run in 32-bit mode?
 
No, you'll probably experience a tad more compatibility, and less laggy/slow flash player

I'd actually recommend this if you plan on watching flash content while on battery. The Flash player is more bloated on OS X than Windows as it is, and by using a wrapper to use the still 32-bit plugin in 64-bit Safari is known to tax many CPUs (just search around here, people's laptops getting hot and fans spinning up when playing flash content). All because Adobe can't get a 64-bit version out the door.
 
64 bit Safari is somewhat faster (particularly on javascript math), and more stable due to running flash player outside itself. Other than that it doesn't matter.
 
64 bit Safari is somewhat faster (particularly on javascript math), and more stable due to running flash player outside itself. Other than that it doesn't matter.

But on the flipside, it uses a wrapper to run 32-bit flash which really eats up CPU cycles. Not good for temp/battery on a laptop.
 
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