I tend to agree with Mick in that the chances of your being involved in a situation where a rollbar's safety properties are called into play are fairly remote. That said, if you are insistent on having one then you have to consider safety, appearance, and if it means a lot to you, originality.
1. Safety: An unbraced hoop or full width bar is a waste of time. Even more ridiculous are those bars that have one solid chassis mounting and the other mounted to the fibreglass body with a fancy surround and tiny bolts/rivets.
2. Appearance: One hoop, two hoops, full width, forward or rear bracing? One rear braced hoop looks fine. Sometimes two hoops can look a bit fussy especially if they're chromed. A full width bar looks the goods in my eyes but some may think it's overkill for a street driven car. For ultimate safety a full width bar
of appropriate height, not some low-slung thing with two rear facing braces together with a triangular cross brace, is the answer. But David, I think you'd regret going for a curved bar over a flat one. Have you actually seen one? They're not nice...
3.Originality: My car has the original Arntz single hoop with a forward bracing bar going down into the passenger footwell. It was there when I bought the car and I like the look a lot. The fact that it's "factory" adds to its desirability, and as a bonus, that brace bar is a huge help when getting in and out of the car!!
Summing up?
Anything that makes you happy as long as it's well made, is of effective height and is properly braced should be just fine ---as long as if you decide on a full width bar, it hasn't got a curved cross bar.
Sorry.