Depends on if you could adjust it to only carry the weight you needed
carried (which depends on the tongue weight of the trailer, not the
amount your tow vehicle claims you can put on the bumper).  If it were
me, I'd buy the smallest one that'll do the job, it will likely have
lighter bars, and that is definitely a bonus when  hitching in the
rain and or cold; you'll have to do it eventually.

I usually use the next higher load rating for the hitch
weight of my trailer. It makes for a better hookup (more
stability). I think you have to realize your trailer hitch
weight is an unloaded weight and, depending on where your
tanks, storage compartments etc. are, can change a bunch. I
would use the 750 lb bars for a 500 lb hitch weight. I
wouldn't use the 1,000 lb bars as it would be overkill and
would make for an unbalanced load when going through a
depression like you see going in and out of some gas stations.
HD in NY

A class III hitch is by definition has a 500 pound tongue weight and
5000 pound Gross Trailer weight.  No need to get anything larger unless
you can pull a heavier trailer.  Class IV: 1000 tongue, 7500 trailer and
Class V: 1200 tongue, 10,000 trailer.

Steve,

I would purchase the 600 pound unit.

With the 1000 pound unit, you will lose flexibility between the
trailer and the tow vehicle. There are situations when you will need
that flexibility. You may also get a hopping or jerking sensation from
the trailer axle as the trailer springs (and the tow vehicle) try to
compensate for the stiff hitch bars, and the trailer contents will be
treated to a more bouncy ride.  

My advice is never try to second guess the manufacturers engineering
department. Take no advice from those who do.

Happy camping,

Ron