The Oregon 300/400t is excellent for paperless caching.
I haven't used the PN-40 so I cannot comment on it vs the Oregon.
I cache with my 11-year old son, and he too just wants to find "treasures" with a minimum amount of fuss and bother. We are not avid cachers, but we enjoy the activity. We used a Magellan Sportrak and paper caches for a couple of years and, while it was adequate for finding co-ordinates, it had little else to offer (and the screen was often difficult to read).
Our new Oregon 300 is wonderful for caching (and adequate but not great for automobile use). Once you figure out how to generate a pocket query at geocaching.com and download it, then you will have the closest 500 or 1,000 caches right in your GPS. If you frequently go to another location (e.g. Grandma's house, the cottate, etc.), then you could generate another PQ for that location and you would have another 500/1,000 caches in your GPS. That number of caches would keep me and my son busy caching for a long, long time.
The Oregon displays everything about the cache - description, difficulty, hints, child waypoints, and even logs. That is fantastic for us, because it means we can just throw the GPS in the car and head out to geocache - no preparation whatsoever required. If we find a cache, we log it and it no longer appears on our GPS geocache list. Then we can either upload our caches found through geocaching.com or do it manually. The whole thing is very easy and intuitive, and the Oregon is fairly friendly out in the field.
The only significant drawback of the Oregon is the screen visibility, but if you follow the hi-visibility recommendations on the Oregon wiki page, and downlad an all-white background, we have not found the screen visibility to be a major problem.
I am also using GSAK to filter and manage my PQ's, so that I only download to the GPS the caches that I want, but GSAK is not necessary for paperless caching with the Oregon.
I am very happy with the Oregon for paperless caching.