Either location would answer for your purposes, but Connemarra would be a lot less crowded than Kerry during the summer months.
We like Clifden too - it's a quaint little market town and the beautiful Sky Road begins just west of the town. It's one of Ireland's great scenic drives, featuring dramatic seascapes around every turn. It's also central for the very pretty Connemarra Loop drive that goes by the National Park (lots of pretty hikes in your range), Kylemore Abbey and the village of Cong - the "Hawk Walk" there at Ashford Castle gets rave reviews from many here on the forum. Potential side trips a bit further afield are the short ferry ride over to the isle of Inishbofin from the village of Cleggan - it's really one of Ireland's better kept secrets. There are boat tours out on the country's only Fjord from Killary Harbor just to the north, and if you're interested in a bit of Irish history the Doolough Memorial near Delphi is worth a stop - it's one of the most evocative places we've ever visited in Ireland. Can google it to see if it might be of interest.
Kerry is a possibility too, though I'd try to avoid the tourist crush in Killarney town at all costs - traffic congestion in the morning when the tour buses start to roll make getting on the road a real pain. Better (I think) to find a place midway between Glenbeigh and Castlemaine to be central for exploratory drives (and hikes) up to Dingle or over to Killarney NP as a day trip. There's also the very pretty Skelling Ring Road that starts at the village of Portmagee - lots of pretty seascapes there with spectacular views of the Skelligs. You can also book a boat trip out to the islands from the visitor center in Portmagee. The Gap Of Dunloe is one of our favorite day trips in Ireland. Can either hike (walk actually) the 7 miles to the head of the glen and back or book a pony trap with a local character for a bit of local color.
You really can't go wrong either way, but my vote would be for Connemarra.
