If I understand your question correctly, I will share what I think to be correct according to minimum code in the US.
If you currently have three wires feeding your garage panel, you should separate the white (common) and ground busses, and drive a ground rod.
If you have four wires feeding your garage panel, the 4th wire will run ground back through the ground wire of the main panel.
Your pulling a correctly sized 4th wire would fulfill the need above for the aforementioned correct ground. (Wouldn't it be easier to just drive a couple ground rods? see below)
You can use a main load center for your garage, it will typically offer more breaker slots. The main breaker should be at or a bit above the rating of the breaker feeding that panel.
A sub-panel in the garage would be smaller, fewer breakers, and less expensive. My personal choice was to economize in areas other than electrical service.
You always need to be aware of how much you are pulling, does it exceed the rating of your main panel when things are being run all over the house and shop?
An extra ground rod will never hurt you. Rent a demo-hammer to drive the rods (code in our locality calls for two - 6' apart) My two rods went into shale/rocky soil in less than 10 minutes. My electrical/hvac dealer rents it out for a day for 35 bucks.
If someone smarter than me (not hard to find) answers your question, feel free to disregard my comments.
