I live in Elizabethtown, so needless to say I am very familier with this region.
I never used Highmeadow since it's only 25 mins. from my house, but we visited it and walked around. To others' point, they really do pack you in there (sardine can). I personally would be chlostraphobic, so as long as you are OK with very little privacy/very few trees, go for it. It puts you right in the heart of Hershey attractions (and the noise that comes with it, as others pointed out).
I have never been to the Elizabethtown KOA, but simply based on its location, you are going to have more room/trees and less noise than Highmeadow. Although you would need to drive into Hershey if you stay there, it's a pleasant 15-20 minute drive in on a 2 lane country road. The other benefit of staying here is, it places you a little closer to the Lancaster attractions (about 30-45 minutes away).
As a sidenote, one amusement in Lancaster that I would HIGHLY recommend is Dutch Wonderland (I believe they are owned by Hersheypark now, but don't quote me on that). It is a fantastic amusement park, extremely clean, stellar customer service and the rides are geared for kids up to about 10-12....adults can go on allot of the rides also. A private campground directly by this amusement park is the Old Millstream Campground, which is the very first place we set our new camper up to become familiar with it. We were there off-season, so although they have trees and a stream that goes through it, you will hear the amusement park noises, but it's very convenient to the park and other things in the area.