Aran Island or Connemara?

That's a tough choice, in my opinion! But, I'd say do the boat excursion to Aran Islands if you only have one day. I enjoy taking the boat to Inishmoor over a plane as you are able to see all the islands from the sea instead of a quick flight over and, quite frankly, I enjoy a good boat ride. Keep in mind, in April the seas could be a bit rough so just be prepared and, as always, be prepared for all kinds of weather--it is Ireland afterall. While I think Cong is charming and Connemara stunning, I'm not a fan of Kylemore Abbey (one of the most disappointing sights I've seen in the whole of Ireland) which is the only reason I'd put this second. Hope you enjoy your visit to Ireland!

I can’t compare because we didn’t have the opportunity to experience both your options, but the boat ride to Inis Mor, the time we spent on the island, and the return trip with the up close views of the Cliffs of Moher was one of our favorite Ireland experiences last May!

I would also pick the boat ride. Connemara is beautiful, and Kylemore Abbey is photogenic, but it is a private school, now, and therefore not as interesting as things you will see on Inishmore.

I would try to wait to decide until close to the date to see what the weather is. I believe the ferries, at least the ones from Doolin, don't run in high winds. I don't know about the planes. FYI: The day tour excursion I did to Connemara gave participants a choice of spending a chunk of time on your own either in Connemara national park or at Kylemore Abbey. I chose to hike in the park after which the bus took us to Kylemore to pick up the others. I had a little time to walk around the grounds of Kylemore (outside the ticketed area). I believe if was with Paddywagon tour.