sifus...
wanna ask....
ATH-AD300 vs ATH-T500 vs ATH-M35...
between this, which 1 is better.....

AD300 outperforms the other two. But it is an open-air headphones, u might want to look at the earlier reply posted by Quazacolt for it's advantages and disadvantages. 

Of course not that loud, but still leak quite alot. I don't think it is usable in public place like library, sure very annoying to the others. And they don't isolate noise well too. U gonna use it as portable phones or only use at home?

Better? in terms of what?

I've tested all of them and I own a AD500 n T500....Among the options you gave, T500 woops AD500 cups anytime....

well, it wont be as loud to the point that people beside can hear the complete full details of the music that you're hearing, however it is loud enough to the point what most laptop speakers can perform at ~half volume. which also means that people beside can pretty much hear what you're hearing, and thats pretty bad imo.

while i havent auditioned the AD300, i've tried the AD700/900/1000. and all 3 of them leak noise out like no other headphones could rofl. to put it simply, 2 damn desktop speakers attached to your head rofl XD

Yes Unless u r getting the M50 or the 700 series, then T500 would be better

audiophile hi-fi headphones, see where the AD series is categorized at in audio technica's page

sound travel through air, if air flow is allowed, wouldn't that help in sound reproduction?
some prefers it, some dont. same concept as full range hi-fi speakers, it moves the air around the room, where some prefer hi-fi speakers, while some may prefer closed can/noise cancelling monitoring headphones or even IEM's

in laymen terms, think of it that open can headphones are sorta in-between your normal headphones and speakers

tldr: its really up to your preference/purpose of your headphones

Yes Unless u r getting the M50 or the 700 series, then T500 would be better