Where in the world are you? I'm guessing somewhere in Europe?
I can find some information on European sites, which give speed data for 50 Hz mains power. Speeds are likely to be be 20% higher on 60 Hz mains if you are in North America. Check the specs from your prospective suppliers.
All else being equal, a lower minimum speed is A Good Thing. Based on the information I can find, the Woodstar and Holzmann both show 180 RPM lowest speeds. The Scheppach shows 230 RPM lowest speed. Highest speeds are at least 2400 RPM for all of them, so this is unlikely to be a factor in the decision-making process.
The Rotwerk shows a minimum speed of 330 RPM. However, because it is a variable-speed machine, it will probably not have the torque multiplication of the other machines and is likely to have relatively little torque at low speed. I'm not a big fan of VS hobby machines for this reason. Whilst I love Variable Speed and have fitted 3-phase motors and VFDs to step-pulley machines, I just don't care for the compromises made to get VS on lower-cost machines. This may be an irrational bias on my part. The electronics that usually provide the Variable Speed tend to be much more prone to failure than belts and pulleys
I have found both 550W (3/4 HP) and 770W (1 HP) motors shown for the Woodstar, though all other details seem the same.
There does not seem to be much detail on the spindle tapers. This may mean they have short, male chuck tapers, probably B16, rather than long, female Morse tapers. The Scheppach seems to have a B16 male spindle. The sparse and conflicting descriptions I have found do seem to suggest the Woodstar and Holzmann probably have 2 Morse tapers (often abbreviated to MT2 or MK2). If you have better information and one or the other definitely has a Morse taper, that should push it up the rankings. The Rotwerk seems to have a 2MT female spindle.
The MT taper is a long taper designed for frequent tooling changes. The B16 taper is designed to more-or-less permanently fix the chuck. With an MT spindle, you use a male MT to male B16 arbor to fit the female chuck to the female spindle. It makes chuck changes much easier (big chucks don't usually play nice with tiny drills). It also means that if you damage the chuck taper, you only need to replace a $5 arbor. If the chuck taper is machined onto the spindle and you damage it, you've wrecked the spindle.
Spindle travel is also a consideration; more is generally better. The Rotwerk seems to have 50mm, whilst the Holzmann and Scheppach have 60mm. I cannot find a spindle travel value for the Woodstar.
Mass is usually desirable in machine tools. The details I can find seem to suggest the Woodstar is close to twice the weight of the other three. If so, it's probably a substantial point in its favour.
On paper, the Woodstar seems likely to be the better/more useful machine.
In reality though, the thing that tends to make the biggest difference to how a drill performs is spindle runout and the degree of general slop in the mechanisms. This is something you can only assess by getting your hands on the machines.
The prices seem to be a long way below anything you might realistically expect to pay for a milling machine so I'm guessing that's not really an option.
