Hmmm...It may be data/local. I just looked on my phone and I have bootanimation.zip in both folders. In data/local the date is Nov 21st. The date on system/media is August 1st 2008.....I def changed my Boot animation prior to the 21st though so I'm not totally sure.
I use a program " Root Tools"(Includes downloadable boot ani's as well as full Root Exp and other options, $2 in market) to install my boot animations. So I have not done it manually, just going off of gained knowledge.
I'm gonna go ahead and say put it data/local first, then reboot. If it doesn't work, then put it in system/media or go ahead and put it in both, doubt it could hurt. Little trial and error never hurt anyone
I'm not a rescue squad dude or anything like that, don't develop or anything. I know nothing about linux/Android Core. Pretty good with Windows but thats about it.
Everything I've told you is just stuff I've picked up reading forums etc. So don't take it as god's golden word. Just trying to help you out.
Oh and like you, I found the ADB **** too complicated to worry about, had no idea what the f*ck it was for a while. For the average user, I don't see it being THAT useful. Just as easy to do it without ADB.