Magic Square are legends sized. They boast some clean robots, great engineering with a ton of articulation. Their material is a nylon sort of finish that is obscenely strong and hard to break but at the cost of "soft" sculpt, molded flash on the detailing, and less flashy paint. There have been some issues with tolerances in their earlier works and some wonky head sculpts. They hit it out of the park with releases like their Prime and Megatron though.
New Age is Legends -1 sized so they're a little smaller scale than MS, near legion sized. They are detailed, built, and painted obscenely well. Their head sculpts for the size are phenomenal and they tend to include a lot of accessories with their releases. Generally they also include a lot of articulation and have fun engineering but skirt a little bit away from strict accuracy for the sake of a better functioning toy. For example, their Datsuns and Jazz have head flaps and stouter proportions than what you would see in Magic Square's strict cartoon style. They tend to shit out every repaint imaginable in very quick succession when they release a mold. Many of their repaints and remolds are ace choices, see their upcoming Galvatron.
You can't go wrong with either really. Plus you can mix and match if you aren't autistic about it. It's up to what you want in your set. I went New Age but find myself wanting some Magic Square releases.