"you" was the ending theme to the Eye Opening Chapter in the original sound novel. Not the vocal, the instrumental. For clarification, the "Dear You" album has absolutely no relation to the sound novel whatsoever.
For the first four arcs, the question arcs, R07 used license-free music and MIDIs from the internet. The answer arcs used music composed specifically for Higurashi, composed by several well-known doujin music artists. dai composed "you", and a fair amount of the other Higurashi music is a remixed version of it. dai also composed "Hope" for Umineko, which is as ubiquitous to Umineko as "you" was to Higurashi.
However, "you" came into the public consciousness with the advent of the drama CDs. Again, the Eye Opening Chapter used "you" as its ending theme song, but it was a vocal version sung by Shion/Mion's voice actress. This proved to be very popular, and soon the "thanks/you" album was released. Confused anime watchers grabbed it and loved it, then grabbed the "Dear You" album and got very confused indeed.
Just remember: "you" is the original, "thanks" is a remix, avoid referring to "you" as "dear you" lest you be mobbed by sound novel fans.