First off: please don't use the free credits from trial accounts. You'll need a MSDN-subscription to do any reasonable mining.
My script includes a autotune-feature: if the VM is instantiated on a physical CPU where a lot of neighboring VMs are idle, the script can use the CPU's cache more intensively, resulting in a better hashrate. If you use a VM with many cores, your chances of having neighboring, idle VMs is less (since there are less cores for neighboring VMs available). That's why the F2-type works best (if you get different results, of course i'd love to hear more about it).

The difference between dedicated and low priority-nodes is that the low-priority-nodes are a lot cheaper (low-priority-nodes may be restarted by azure at any time, however, for our mining-application this does not matter).

Since you want to burn your 200$ as fast as possible (i assume you are using a trial account), you should create as many vms as possible. A pool with 10 low priority F2 VMs (20 cores) will consume about 150$ per month, so you'll need more than that. You'll probably have to request a quota increase from the azure-support.