If you want non-stop from ORD to SXM, your only choice is UA. If you are okay with one-stop, same day routings, you could add AA, DL, T3 and US into the mix. If you are willing to do an overnight somewhere, either CO or AC would be more comfortable in the front cabin. AC operates a 767-300 from YYZ-SXM. The entire a/c has seat-back IFE with a good selection of TV, film and audio, and the front cabin is equipped with lie flat pods. This may not be a big deal on a 5-hr flight, but the return flight arrives in YYZ at 10pm meaning you can get a head start on your shut-eye.
If you are planning a 2-4 day trip, I would go for UA for the much shorter total travel time. For a week long trip... either CO or AC or perhaps a mix of AC on the way down and UA for the return without the need to overnight in YYZ in the winter.
I forgot to mention that Air Canada operates to/from SXM on Saturdays only. I've been to SXM four times this year on business and have at least two more trips planned before the end of the year.
I use AA for ORD-SXM once in a while. Recently the only real option is connecting through MIA. In reality, don't get too excited as it's basically 2 3-hour domestic first class flights most likely on a 757. The seat is larger and you get food and drink. It's branded as business class so as not to confuse the service with international first class. Certainly more enjoyable than sitting in back.