What? It depends on what you are working on. What are you working on? Aircraft? You need 12 pt.
I don't believe 12pt sockets are any weaker or round off fasteners any more readily. I think they don't handle corroded, under sized, or slightly rounded hex fasteners as well as 6pt.
I still run into 12pt metric bolts on some cars. Last time was caliper bolts on a Land Rover discovery (LR3). So I have one set of 12 pt metric in 1/2 drive. Otherwise all my other sockets are 6pt.
Also, when I upgraded my craftsman tools to Snap On, I concentrated on metric sizes only.
One more bug in your ear. You will need basic shallow sizes for most jobs, almost regardless of what you are working on. Beyond that, consider island hopping to new technology. For example, I don't own a breaker bar. I use high strength ratchets like the SHLF80.
I have only one set of deep sockets. I prefer semi deep- I find them deep enough and they fit more places. I use swivel impacts on my 3/8 gun. I have no universals. I use swivel chrome on flex head ratchets and find I can get anywhere with that setup.
That's brings another point to mind- I recommend starting with flex head ratchets, and stubby or stubby flex. These apmay not be preferable for every job, but I find they offer increased utility.