I have a 5050lt, the 5100 series is the newer model(more ppm, ect).
This is the way it works:
If the program tells the printer which size of paper is needed(i.e PDF), it will automatically pull the correct size. If the program does not(i.e. Swiftview), it won't. The solution is to load legal in tray 1 and letter in tray two. Then go to 'properties' and set the size for each tray. If the program tells the printer the doc size it will pull automatically, if it doesn't, it will pull from tray one and print on legal.
There are some printers that pull the correct size based on the document content, but they are usually more expensive. Using the printers you listed, the 1320tn runs about $600 and the 5150LT runs about $300(not taking into account rebates, internet specials and the like).
I have a 5040 and was looking for a lower tray, and found the 5050lt on ebay for $86 including shipping. The cheapest I had found for the tray was about $125, so I figured if the printer doesn't work, at least I got my tray(not to mention the drum unit). Fortunately, it works great and now I have a back-up printer. I am very happy with the Brother printers and have no reservations about reccomending them.