@RedPandas: I’ve been to both Oahu and the Big Island twice now. My first trip to Hawaii, we went on the Pride of America, so we got to see all the islands for a shorter amount of time. The second time, I went to the Big Island and stayed at the Marriott in Kona before heading to the Hilton Hawaiian Villages in Waikiki.
My advice is that it depends on what you are looking for. I tend to be someone who needs activities and entertainment at night. If you are a person who loves to relax on vacation and are perfectly content with a book for x amount of days, then the BI may be great for you.
If I’m correct, the places to see in the BI are Kona and Hilo. I visited Hilo on the cruise (though I did not stay overnight, so I can’t make a judgement about hotels). To be quite honest, from what I could tell, it was boring. I found myself back at the ship after about 1.5 hours. There is a black sand beach (since the island is mostly volcanic rock), and a tiny town area to eat and maybe pick up some trinkets, and I would not go back. The only reason Hilo was a port of call for the cruise was the volcano, which was not active at the time due to the tsunami in Japan (I went in Aug of 2011). I would do some research if you hope to do a helicopter ride over the volcano as an excursion.
Kona, is beautiful, and has a very laid back vibe to it. I loved it the first time I went so much I swore that it was my favorite place in Hawaii, but the reality is that there isn’t as much to do at night than I had initially thought. Additionally, the resort I stayed at is in Waikoloa which is almost a bit sterile for my taste- I am not a fan of strip malls on vacation. This has a very different feel than Kailua (the beachy town I was talking about earlier). The beaches are gorgeous, but there is very little nightlife in Waikoloa. It’s definitely a relaxation spot. We also went to visit a coffee farm, the Kona Longboard brewery and went snorkeling while in Kona. I felt that there was so much to do during the day and not as much at night, so I might rather spend time in Oahu. Again, personal preference.
Since you’ve been to Oahu, you know what to expect. I would suggest that if you are looking for a more lively atmosphere (but also more crowded!), then Oahu is your best bet. If you don’t mind a very quiet night, I would visit Kona since you’ve never been. Oh, and to answer your question, rent a car for sure!
I hope this helps! Let me know if you have any questions, and enjoy!!!