I think the wreb is easier than crdts. i also think the wreb is going to be around for some time but the crdts may disappear because of the new ADEX exam.
As far as training I dont think you really need any specialized courses.
If you do wanna prepare yourself better I suggest you read the exam manual carefully, they pretty much tell you what the criteria for passing each section are. Also if you haven't cut teeth in a while, you can buy a compressor ( any compressor will do, doesnt have to be the expensive dental one) and a handpiece , have them hooked up by any dental tech and practise on plastic or natural teeth all in the conveniece of ur home.
The duggan courses are very expensive and geared more towards the CA bench exam which is a much harder exam than any licensure exam out there.
Also make sure you have reliable patients for your exam (preferably friends or family) so they will show up for you.
Good luck.