I have used all knuckle systems through the years of me layout and have found the following:
USAT knuckles are good except for when in high tension situations ie a heavy consist going up a grade. Then they can separate fairly easily even the ones on the aluminium streamline cars, I know, I have watched one of my prized Rio Grande 'Ski Train' cars run back down my grade at increasing pace!)
Bachamnn knuckles great for narrow gauge 1:20.3 scale consists as long as not heavy consist going up a grade. They also look pretty good especially the metal variety. The smaller plastic type used on some of the Bachmann 1:29 scale rolling stock are not so strong. They did start to use the 1:20.3 scale ones on more recent editions of 1:29 scale (last few years) and these can, if you are finicky, look a little out of scale (but that is true for most makes except possibly for the Kadees)
Accucraft knuckles look the biz BUT not strong enough if....yes you've guessed it..on a heavy consist going up a grade.
LGB knuckles very good if you can get them but they too are a little weak on.............a heavy consist going up a grade
Aristocraft knuckles are the strongest couplers ( I have a very heavy grade and have triple headed diesels hauling up to 6 aluminium streamliners coaches up that grade. I have never (oh dear ...tempting providence again) had a breakaway using the Aristo couplers. The only problem with them is that their release bar does hang a little low and can be nudged if you have a large piece of ballast or heavy twig lying in the centre between your rails on top of a tie. A good idea is to cut about 3-5mm from the bar if your track could be affected by 'lumps or bumps'.
Interestingly Kadees are great in appearance and operate in a prototypical method but they are troublesome if not looked after (corrosion, they rust pretty easily especially the hanging lift spar etc if not protected with corrosion inhibiter). I have had breakaways on long consists.
If you have a layout without major grades and you do not try to haul as many cars as you can then all of the manufacturers knuckles will serve you well. Some varieties are compatible even if you have to drop one car knuckle into place over another when the knuckles are closed. BUT it is best to standardise, as mixing and matching will eventually lead to tears. If cost is the concern and you cannot get access to freebies then Bachmann could be the cheapest option unless you buy rolling stock new and get the manufacturer's knuckles in the box!
If your track is very 'undulating' or just plain uneven, then perhaps knuckles are not for you because even the best will eventually ride over each other or just unhook because of differing heights of the cars as they navigate the undulating track. Hook and loops are mainly unaffected by this.
The comparisons above are based on experiences where the track is both undulating and non-undulating and running through curves of R3 and wider (the length of coupler bar and amount of swing is also important here). Using a test pair of knuckle endowed cars is a good quick way of testing the general 'even-ness' of your track before you investigate further using straight edges and spirit levels.
<message edited by beavercreek on 16/07/10 12:53 AM>