Centre classes would be very rigorous. Loyola classes would be rigorous but the overall expectations would be a notch below.
Baylor is a great choice if you are conservative and a faithful Christian. The competition among premeds there is fierce, but it’s the top college for Baptists so if you are, it’d make sense. And if you’re not religious, it wouldn’t really make a lot of sense to choose Baylor over your other options.
Union is a college known for sciences, especially Engineering. It’s heavily Greek, something I wouldn’t recommend for a Premed since during the Rush/Pledging semester GPAs drop significantly (as per research, especially at fraternities). If you don’t join a fraternity, there won’t be a problem, but you want to see what sort of activities are offered on weekends (free films, outdoors adventures, trips, concerts…?)
There is no “better for premed” in the absolute. The better is what is the best fit for you + where you rank in the top 25% + have the best value. Without costs and without your stats, we cannot advise which ones would be “the best” for you.
All premed programs are difficult. Some are, in addition, very competitive. In general, would-be premeds are more successful in a more collaborative environment. Costs and fit also matter: it’s harder to be high-performing if you’re unhappy.