Can't really say which is better, having never owned a Bredli, and those that I've come in contact with were absolute stunners, with awesome personalities and great colourations.
However I have owned a number of Diamonds, purchased at varying ages from hatchies (1 month old), to 9 month olds, to Yearlings, to 2 year olds etc.
The hatchies I've purchased were great, good strong eaters, active, never aggressive.
1 out of 2 9 month olds the same - still have him and he is without a doubt the best natured snake I have come into contact with, the other developed a bit of stress during feeding around the 1 year mark, but has since come good with a change in feeding routine.
3 yearlings - all from a Captive Mother and Wild Caught father - breeder got through foster care - 2 nice, even tempered, 1 snapped twice after not being handled for a week.
Both 2 year olds are as placid as they come, and you really couldn't ask for a better snake. I guess as with any breed you have good and bad ones (and by bad ones I don't necessarily mean the snake as I believe they are all good, they only react to poor handling practises and artificial environments etc.).
If you were to get a Diamond, I would recommend buying from a reputable breeder, a snake which has been well looked after, is an established feeder, is shedding well, and would recommend examing a fecal sample for worms. Check the skin for mites, check the tail tip for shed retention, eye's for retained eyelids and mouth for excess saliva. Check for an irregular clicking sound or any kind of wheezing in breathing, and for active tongue movement. Temperament should also be considered although snappy juveniles can turn into even tempered adults. Hope this helps paint the Diamond side of the picture for you.