Most all the lithium ion tools are fine if they meet your purpose. A major consideration is where are you going to get batteries and parts 10-20 years from now?
Probably from DeWalt and Milwaukee.
Dewalt is still supporting their 18v nicad line with new batteries and bare tools if you choose to go that way rather than upgrade. Those tools were first introduced in the late 90's. They don't offer much more than the basic tools, but the old nicad packs could not adequately power many of the tools available today for the lithium packs. My first 20v tools are nearing 10 years old and the tools/batteries are still in use and going strong.
Both are great. Personally I went with DeWalt 20v and Milwaukee M12.
So far so good... but the only defective tool I have received has been from Milwaukee.
If dewalt expanded their 12v line I would be interested, but it would take awhile to catch up to m12. It's hard to go wrong with all of the innovation coming from both brands. For the bigger tools I just went with the best deal available at the time.
The only thing that differentiates the brands is the flexvolt feature of the Dewalt. Otherwise, quality is the same among major brands.
Milwaukee seems to have a slight edge in mechanics' tools