I have no experiance with the Harbor Freight Brand. I do own both Dewalt and Craftsman tools. Some are brushless some not.
As has been said the brushless seem to get more run time per battery.
I have adapters to be able to interchange the battery platforms. I have also ordered adapters to convert all my old 18v Dewalt to the 20 v batteries. They work great, more power and run time that with the 18v.
If you can wait until black Friday / Christmas time Dewalt always runs a special buy 2 batteries and charger get a tool free. I bought 2 4ah batteries and chargers for about 129.00 got the jig saw free. The Angle grinder was an option but I already had the Craftsman.
I got into the Craftsman line because my wife wanted a cordless light weight weed eater. We now also have the hedge trimmer, limb saw, leaf blower, angle grinder and and drill. She has also hinted that she wants th R/O sander that they have. I am not the only tool junkie in our family.
If you decide on the Harbor freight I would be very interested in your thoughts. They make adapters to be able to use the Dewalt batteries for the H/F tools. I my experience make sure that you get at least a 4 ah battery for anything except a drill.