Buen Camino, Anthony!
I walked to SdeC last July via el Norte and el Primitivo; the temps in midsummer were consistently 80s and 90s F, but in May the climate will be considerably colder and quite damp. Even in Galicia, you will encounter quite a bit of road-walking, so be prepared with comfortable shoes. Some people would opt for the most waterproof option, but keep in mind that lightweight trailrunning shoes will dry much faster, as long as ankle support is not an issue for you. I brought my trekking poles and was so happy I had them--many on the camino prefer a single wooden walking staff, but I like getting as much weight as possible off my knees and ankles.
One nice aspect of walking the Camino is that you needn't worry about carrying tons of gear. Most people do not bother with a tent and find it to be extra weight. Unless you have made plans to camp often along the way (which is how I planned my itinerary), leave the tent at home. As long as you finish your walking fairly early in the day (start as early as possible in the morning), you should have no trouble finding beds in the albergues. Also, although I can understand why people carry stoves, this seems like overkill to me, since there are myriad opportunities for hot meals as the trail passes through so many villages and towns. (+1 on the cafe con leche!)
In terms of insulating layers, I would opt for the down jacket, simply because it is lighter and more packable than the Arcteryx softshell, and has better warmth:weight ratio. Just make sure you have a good waterproof shell to keep the rain and heavy mists and fogs off. The Galician "lluvisna" is legendary! Many use ponchos to cover themselves and their packs, but I was quite happy with my rainshell (better than a poncho in the wind) and a plastic trash bag as a pack liner (cheaper than a fancy pack cover).
One more thing, keep your eyes out for the new movie "The Way" starring Martin Sheen. Not sure of the release date, but it's a father-son story about walking the Camino--looks a bit corny, but I'm sure I'll be seeing it when it hits the theatres. Suerte~KM