Mercury developed dags have higher contrast and greater apparent resolution than becquerel developed dags. There are some obvious reasons to do becquerel developed dags - safety and convenience chief among them. If you are planning to teach yourself the process, begin with becquerel until you understand the process. There are various learning curve bits you'll pick up that will apply to mercury developed dags, without the hazard or the expense. One of the keys to doing mercury developed dags is using bromine water, which is highly caustic and dangerous. Skip using the bromine until you know the process better.