Next year we would like to stay at one of the HGVC oceanfront resorts in South Carolina for a few days after a visit to Clemson. Since we fly in/out of Atlanta, Hilton Head appears to be the closer location.
Is Ocean Oak at Hilton Head the best choice versus the HGVC resorts at Myrtle Beach? If so, is it feasible to book 9 months out in mid April for a weekday stay? We prefer tranquil beaches over commercialized beach boardwalks.
What activities exist at Hilton Head in mid-April? (we don't golf)
Is it feasible to take day trips to Savannah or Charleston?
If you prefer tranquil beaches over commercialized beach boardwalks (as we do) then Hilton Head is the clear choice. Commercialized beach boardwalk is the literal definition of Myrtle Beach.
We've been to Hilton Head the last two years in April with our Marriott ownership (Marriott's Barony Beach Club in 2017 and Marriott's Grande Ocean just last week). We spent a week at HGVC Ocean Oak last September. Ocean Oak is on a great, wide stretch of beach, right beside Marriott's Grande Ocean.
As GT75 said, April HHI weather can be variable from cool to great beach weather. In 2017, it was sunny and upper 70s/low 80s all week, with no rain. Our most recent 9-night stay had four sunny days with temps 70-82, a partly cloudy to cloudy day with temps in the mid-70s, and four cool sunny/partly cloudy days with temperatures in the 60s. Those cooler days, with brisk winds, required sweatshirts or jackets in the morning and evening. The only rain was one evening on our third night from about 6pm until after midnight. Heavier rain moved in for a couple of days right after we left.
One thing you can do regardless of the weather in HHI is eat. The restaurants are outstanding, and most are locally owned independents or local multi-restaurant groups. Very few "chain" restaurants. Do a search on the Marriott forum for Marty's Hilton Head Restaurant List for some great restaurant reviews from a TUG HHI expert.
If you ride bikes, HHI has bike paths that cover the entire island, and is a popular activity. There are also many water activities such as boat cruises, sunset dinner cruises, dolphin/wildlife cruises, parasailing, kayaking on the canals/lagoons, etc. You said you don't play golf, but if you would enjoy watching the pros play, mid-April is the PGA Tour's RBC Heritage golf tournament (in 2019 it is April 15-21). That tourney was one of the reasons we went in April in 2017 and 2018.
There is shopping just off the island at Tanger Outlets, several miniature golf places (much more limited and discreet than the mega-mini golfs in Myrtle), and history tours. So plenty to do in April. A day trip to Savannah is doable, but since Charleston is two hours away, you would spend four hours of your day going and coming.