I've tried both. I am currently a fan of putting it as close to center as possible. I've had it at the top, and the Kossel I built ended up being top-heavy. I used 1515 openbeam, and it's XL sized, so if you have some reinforced 2020 beams or better, you may have a good outcome from on top. it means you have to handle all your filament on top...
on the other hand, I built an arm off the side, so as to get as close to center, I have been growing less of a fan of that option too. it sways and has snapped off a couple times, but I get much better results and less hassle with the actual Bowden, if only I could shorten the length a little more.
if yours is the "floating" extruder with the nylon fish cords, that looked quite the interesting, however I imagine that's not your source of vibration; it's most likely your main motors, or belt tension. try adding simple 1/2" stick on rubber feet you can get at home depot. I did that and it was night and day... and I run my belts tight enough to strum a low d chord...
So now, I am looking into some sort of slide-mounted extruder, thereby having it as close to an arm, and shortens the Bowden length. plus, it preps me for a diamond head 3-filament extruder.
