In a broad sense, mills can be classified by size. The smallest classes are usually rather precise and are used for things like cutting small parts, model making and carving wax for jewelry casting.
Sieg, the Shanghai Industrial Company, while the RF45 is from Rong Fu, a Taiwanese company said to make a higher quality machine. Note the weights in pounds along the bottom. The micro mills (like I have) are very easy to live with. You can lift them to vacuum under, if you want. An X2 class machine, at around 200 pounds is something you don't want to move very often, while the X3, RF45 and larger machines are something you want to move once, with serious planning. Maybe build your house around them. As an anonymous commenter said in that linked previous post:
A 700 pound mill not on wheels cannot be moved with body English. If you put it on wheels on a sloping concrete driveway you want a pulley to pull it up the slope. You can keep it from tipping over with your hand, but if it starts to go you can't even slow it down, jump away or have your safety buddy call for the jaws of life.As Make Zine said, Sieg machines like X2s are one manufacturer with many brands. Not only that, the same machine is tweaked by some of the American sellers for different features: changing the table size is very common. As always it's a bit more complicated. There are other lines of machines, like Grizzly G0704 which is a little bigger and little more powerful than the X2 clones. The G0704 is a representative of yet another maker's product, a BF20, sold by several suppliers.
By now, I can hear lots of people saying "but what do I need?" Again, it depends on what you're going to do. If you want to carve out 80% lowers, you can do that on a good drill press. I've done it on my Sherline. Any of the X2 mills will do. I'm simply not sure what tooling is needed to do a blank forging, a "0% Lower", or to make a receiver from a block of metal. The 3990 has 10.6 inches of Z-axis travel and remember that any cutting tools or holders (collet or drill chuck) will eat up some of that. That sort of work may need more Z-axis but it might be that those cuts can be done on a large lathe.
Right now, I'm leaning toward this LMS 3990. The Griz G0704 is a strong contender, though, and I'm still going back and forth between them.