I've been to gyms that have both. I'm not an extreme fan of either one, but that's just a personal preference. I'd say neither is better, though you'd be less likely to "hyper"-extend (over-arch and come above parallel) with the 45-degree one.
With the flat one, if you can find a way to target your hams and glutes more, I'd say that's a bonus. Mind you, it's no substitute for a GHR, but I suppose it would be useful for some variation.