As was mentioned it really depends on how important having a perfectly flat load floor is to you. If you have to ask then my guess would be the minor height of the mounting rails of a standard kit is very unlikely to be a problem for you and I don't see why you'd want to spend that much extra money for a flat system. Standard rails don't present that much of a problem and I would think they might matter only to someone who uses the bed for other loads on a very frequent basis. In reality if you're like most people you probably won't even be removing the hitch that often.
With regard to strength, you're not gonna break it.
The things I would look for is easy operation and a reliable jaw design (in terms of locking, and it being obvious when not fully locked.) A human-error accident is infinitely more likely than something like over-stressing the hitch, and will really ruin your day.