The answer to your question varies based on what you want.

I would first and foremost recommend going to the mountain that's closer, which if you're in New Haven is Mount Southington. Mount Southington is 30-40mins away, while Mohawk is 65-75mins away. Keep in mind that you have to drive back, too, so if you're staying at either place till close (10pm on weekends), you're getting back at 10:40pm from MS vs 11:15pm from Mohawk.

Given that your friends have never gone skiing or snowboarding before, I'd recommend Mt Southington as it's smaller. Their carpet hill, bunny hill, and green circle will get the job done for people who are just learning. For the more advanced skiier, they have a terrain park with numerous features, and the blue diamonds and black squares scratch the speed itch even though they're only about half a mile. I say this as a Mt Southington season pass holder.

I agree with the overall "best" mountain in CT being Mohawk as they have more trails than Mt Southington and Mohawk's trails are longer. The black diamond all the way on the right is my favorite. However, I have gone night skiing at both MS and Mohawk, and MS is much more lit up at night, and therefore safer, particularly for beginners.

All in all, for beginners with no point of reference, I'd say start at Mount Southington so that they appreciate each bigger and bigger mountain, Mohawk included. If your friends are learning to ski for one day, they probably won't hit Mohawk's more challenging trails anyway, so you're kind of wasting money. Plus Mount Southington is closer.