Max, I and several other people on the forum have used the LongMill for lots of aluminum parts. With careful setup I have gotten accuracies on 3/8" aluminum plate within ± 0.001". Keys for both the Mill One and the LongMill, and really any other CNC, are 1) Proper design - Minimize extremely thin walls, confirm the diameter of your cutter with tests, not just by mic, and measure the thickness of your stock carefully; 2) Good stock hold-down - Secure in all 3 axes, and 3) Use of correct settings. This is where you optimize the machining settings for what’s most important to you - cut quality, cycle time, accuracy, surface finish, etc.
Because of its table size my choice would be the LongMill. You can easily secure a 24"x24", really 30"x32", sheet of aluminum and cut, depending on your outside part contour, a grid of 6 or 7 by 10 or 11. Universal G-Code Sender has a cool replicate feature that lets you load one program and duplicate it in an array to cut many parts at once. There are web sites that will arrange your parts on the available sheet to use the plate most efficiently. If you’re a Fusion 360 user, it can now do the optimization internally. This combined with the use of parametric design makes producing large numbers of parts much simpler.
When you get ready, no matter which machine you choose, let us know. We can probably save you some head-scratching, although that is often the best part.