I am a first-time builder, and decided to go with all the stock harnesses that came with the individual kits as well as purchase the pre-wired WH-00125 main fuselage harness. All of those harnesses are actually made by SteinAir for Vans. I had decided early on to use SteinAir for my panel, so that helped my decision to go with all the stock harnesses that Vans provides for the RV-14(A). I added a few wires here and there (e.g. pitot heat wires, multiple RG400 runs for antennae), but nothing major. I liked the idea that I would be installing wiring as I built each sub-kit, when access was easiest.
When I installed the panel and programmed the avionics, it was nice to be able to reverse wiring when needed at multiple connection points, which the stock harness affords (you can change wires at the intermediate connectors, and not just behind the panel, which is helpful at times). As I mentioned, I went with SteinAir, which then wired my panel to mate with the large D-subs and molex connectors that are part of the WH-00125 main fuselage harness. The whole experience was nearly seamless - I only needed to tweak a few items.
BUT, I have several friends who opted for Aerotronics, and are very happy with their choice. Aerotronics provides all the harnesses, so you do not need to purchase the WH-00125 separately (or, from what I understand, the individual sub-harnesses that are part of the Emp and Wing kits). At this early point in the game, you might call both companies and ask about their wiring styles/philosophies.
I also learned early on that building my own harness (or panel) was not in the cards for me. Even with a prebuilt harness and prebuilt panel, one still has to terminate over 100 connections, confirm all the wiring diagrams, etc., so you'll still get plenty of avionics experience. Having completed all of that, it was more than enough for me.