IMO the most useful batteries to get for most things m18 are the 5.0. They are already offering over 2x what the old Nicad batteries would offer in a similar size, and the m18 3.0, 4.0 and 5.0 are physically the same size I believe, so you might as well go for the larger 5.0. The absolute best place to buy Milwaukee batteries separately are eBay. You can find them in original sealed clamshells for reatial sale at half the price of retail.
My most recent purchase was the newer m12 6.0. I got it for 1/2 price, in a sealed clamshell package.
The 2.0’s are great for their size, and they would offer the same (a little more actually) if you compared them to something like the old dewalt 18v Nicad stick packs. For a little more $ and weight you can get the 5.0. More runtime, and the higher amp hours will generate a little more power and torque in the tools.
When or if you ever get other tools that are more constant use, or power hungry, - like a saw or a grinder, the 5.0 is a much better option for things light that. If you already have a 2.0, then you already have the option to keep some size and weight off the tools if you need/want it for a specific task. IMO - its a better option to go for something bigger now.
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