What would you choose, Dingle Pen or Ring of Kerry? Will only be able to do one of them. :(
I would choose Dingle Peninsula. We have done both. It was rainy and foggy when we did the Ring of Kerry and glorious blue sky when we did the Dingle Peninsula so that may be coloring my judgment but the views from the Dingle were gorgeous.
Ditto the Dingle! On our trip around the Ring of Kerry we were disappointed that the ocean and the great views it typically affords was out of sight most of the time. And Dingle Town is a great place to use as your base.
I agree with Mary and David. I too have been to both and the Dingle Peninsula is more dramatic than the Ring of Kerry and a much shorter drive. I would also sleep in Dingle for two nights and enjoy the town and pub scene.
I'll put in a plug for the Ring of Kerry. Especially if you get off the beaten path the ROK is much more diverse and interesting. The first time I was in Dingle was in 1989 and it was great. But by 2018 I found it to be crowded and a little "fake" - the whole dolphin bit put me off. Yes the ROK tourist stops on the bus route play canned Irish music but if you like to walk or hike there are breath taking views and challenging vistas.
If you'll have a car then I agree with Trotter that the ROK, especially the northern part, would be my preferred choice. Both are wonderful, but during the height of the season (June to Sept) both Dingle and Killarney tend to get overrun with day trippers and tour buses. It can be a congested nightmare just trying to get out of Killarney unless you get an early start.
The northern half of the ROK includes the beautiful Gap of Dunloe ... the prettiest little glen in Ireland for our money ... where you can bike, hike, or take a jaunting car to the head of the valley and back and enjoy picture postcard views the whole way. Just up the road is the exquisite Skellig Ring Road - a detour off the ROK and which features gorgeous views of the Skelligs and other attractions.. Tour buses still don't go out there so it's a much more mellow experience with a car than jousting with them on either Slea Head Drive or the ROK.
If relying on an organized tour then either would be good. Though crowded during the day, both Dingle and Killarney become a lot more pedestrian friendly after around 5 PM when the last of the day trippers depart. Since it doesn't get dark until very late during the summer you'd still have lots of daylight for exploring the National Park at your leisure - on foot or by bicycle.