Like most tractor headlights, they're sorta useless if you have a bucket up in the way, but I have to say that when not blocked, the new LED headlights (both high and low beams - the 20's actually have high/low positions) are noticeably brighter than the 3510's halogens.
As to the joystick, do you believe you might someday want to install a 3rd function kit on the 3510? (or are you planning to do so now?) If either is the case, are you OCD? Will wiring for the two buttons that is not hidden in the 3510's joystick tube bother you? The wiring on the 3520 joystick is completely inside the joystick and is not visible. Your call; $4k is not insignificant.
If you are not planning on installing a 3rd function kit now, I'd suggest reconsidering that. It is less costly to install a 3rd function kit at time of purchase, while the tractor is already apart during setup (i.e., cheaper than an install later, which requires disassembling the tractor to get to the hydraulic fittings on the axle housing. My dealer only charged $300 for the install, which in my mind is pretty good as it is rather involved -- not hard, just time consuming. I had intended to do it myself, but he mentioned that a me-installed kit likely would not be covered by Kioti under warranty.
One other thing about the joystick: on my 3520 cab (not sure about the open 3520), there is third additional button on the left side of the knob that allows you to momentarily raise the engine rpm to handle a high load, without having to leave the manual throttle at full tilt all the time.
HTH!