One of the best ways I've found to guesstimate crowd levels is to check hotel rates online. You can pull up an online calendar with nightly rates for most of the biggies (Mirage, Luxor, Circus Circus, etc.). The more expensive nights are when they have big conventions or other events that bring large groups of people into town. A couple of things I can think of that will bring in the crowds (and jack up hotel prices) are the consumer electronics show in early January and Valentine's Day weekend. And, obviously, weekends are busier than during the week. Check the calendar for the hotel you want, figure out your dates, and then make sure SW has availability for your free tickets on those dates.
Weather is less predictable. We've been in January when it's been sunny and warm (60's, maybe 70), and we've been in April when it was cloudy and so unbelieveably windy that our hotel pool was closed for the entire 4 days we were there. But in the winter we've never needed more than jeans and a sweatshirt or light jacket, and usually can get away with shorts and t-shirts at least part of the time.
Good luck, and have fun!