I went the Dewalt route, but did some research into the brands before entering the ecosystem.
I was extremely interested in Milwaukee's M12 line, for it's light weight and compact size. Ultimately I came to the conclusion that Milwaukee gear is really expensive in NZ, and I couldn't justify the cost for DIY use.
Milwaukee has a reputation of selling some of the best battery on the market. Compared to dewalt, it has a wider range of tool's, and many specialist tools that would suit electricians & plumbers.
Milwaukee has stuck fairly rigidly with it's 12v & 18v lines, with high power needs handled with specially marked "High output" 18v batteries.
This differs from dewalt (which has it's 15 cell flexvolt packs and 54v tools to take advantage of these cells, while maintain compatibility with 18v tools (by series / paradell switching 3 string's).
This also differs from makita, which either uses duel 18v packs in it's higher drain tools, or has a recently introduced 40v Max (36v nominal) line.
For me the fact that Milwaukee isn't carried by either of the large DIY hardware stores (Bunnings/miter 10) also counted against them.
I follow a dewalt tool enthusiast page. Relatively common for people their to run two battery systems, using dewalt for bigger tool's, and using Milwaukee's M12 for smaller stuff (and to fill gaps in the dewalt lineup).
However I also entered the dewalt 12v lineup (not stocked in NZ), for my second drill, as the dewalt drill was cheaper, lighter, and more powerful than the M12 one, and the hand grip isn't super fat, so my child will be able to use that drill.
The Milwaukee's surge impact driver, and their assembly tool looked especially good (but Dewalt now has their own assembly tool).
On Makita, it is by far the most common quality tool brand in NZ, which has it's advantages. Big range of tools (i.e. 15 current of coming soon options for line trimmers/ Brushcutter), especially for carpentry.
Unlike the other brands, Makita serves both low end and high end users under the same brand name. Means you need to look into where each tool falls in the range (and avoid the entry-level stuff for say trade use), but this does have the advantage of, in the situation you want a pro grade drill, a mid range circular saw and say an entry level grass trimmer, that you can get both on the same battery system.
Milwaukee is owned by TTI group, and their mid range stuff is sold under the AEG brand, and their DIY stuff is sold under the Ryobi brand (batteries all incompadable with each other). Same deal with standly black and decker owned Dewalt. Dewalt is high end, Stanly fatmax (craftsman in the usa) mid range, Black and decker low end.

As others have said, Makita batteries commonly fit aliexpress tools.
If you went the Makita route, you would need to choose between their tried and proven 18v range, and their new 40vmax range. Latter means no option to run 5 cell packs on smaller tool's, but should mean a bit more power & efficiency.
Dewalt & Milwaukee both tend to have better deals on starter packs of tools than Makita. Ultimatly this is what got me into the dewalt ecosystem. To good to be true drill / driver / flexvolt battery / charger deal on black friday a few years back, and I now have a whole tub full of black and yellow tools.
Makita has some of the cheapest (retail price) batteries on the market. $169 (bunnings) for a 5Ah, Dewalt is $204 (bunnings), and Milwaukee is $269 (placemakers). Can't comment on other brands, but new Dewalt batteries are available fairly cheap on online marketplaces, as people sell the batteries that dewalt gives away in "buy x skin and get a free battery" promo's.
Technofreak: I chose Milwaukee in the end. It was a close run thing. I went to buy with no real preference.In the end it was the deal that was available on the tool plus charger and batteries that swung it.
As far as I can tell both Makita and Milwaukee have an equivalent range of tools available here. The range of tools and accessories available for both makes was the main reason I had short listed both brands.
Just saw this. I am sure you will be happy with your purchase. some of the best tools on the market. And many of the chargers in kits also have a M12 charging bay, which opens up access to that range to tools too. Stuff like plam nailers, soldering iron's, Caulking gun etc.

