Going to Maui around the middle of January. Which area is the best for beaches and sunshine. Considering the Ritz or Renaissance. Thanks for your input!

If weather is your primary criterion, this one is beyond debate. Kapalua is cooler, windier, rainier. Beaches are more plentiful in Wailea as well.
While the Ritz is a nicer hotel, the weather there can kill a great vacation. True, you can stay at the Ritz and drive to Wailea if the weather is bad, but you'll probably spend a good deal of time in the car.

Better weather in Wailea, more upscale, remote, less kids. Kapalua is more crowded, however January should be ok. But if weather is your concern, as it would be mine, Wailea hands down.

Kapalua's not more crowded than Wailea(!?). It's Kaanapali that's crowded.

Thanks for the input guys! Sounds like Wailea.