Factors to take into account when choosing a scrub plane include quality, convenience of blade adjustment, and whether you prefer working with metal or wooden planes.
Metal vs. wooden
Both metal and wooden scrub planes usually lack mechanisms for adjusting the depth of the blade. You generally have to make the adjustments by hand, or with the help of a with a small mallet or hammer.
However, a metal scrub plane always has a lever cap with a wheel nut, or knob, for clamping and releasing the iron. This can make matters a little easier when adjusting blade depth.
The set screws in the sides, or ‘cheeks’, of the body of a metal scrub plane can make it easier to adjust the lateral, or horizontal, angle of the iron.
These advantages mean that if you’re only just starting out as a woodworker, a metal scrub plane might be the best option.
If you’re more experienced, or if wooden tools appeal to you more than metal, the trial and error of achieving a perfectly positioned blade in a wooden plane with a wooden wedge and no adjusters might be part of the overall pleasure of woodworking.
As with most things, if you’re willing to pay more, you get more features. Wooden scrub planes with both blade depth and lateral adjustment are available, but at a price.
Narrow means easier
The narrower your scrub plane is, the more easily it will cut into the wood, so unless you’re looking for a challenge to your arm muscles, narrower – about 38mm (1½) inches – is probably better.
Iron is all-important
The iron is an all-important consideration. Scrub plane irons need to be strong because of the deep cuts they make, so the rule is thick and hard. A blade made of A2 steel and around 4mm (5/32″) thick is best as it is unlikely to flex and cause chatter (see Possible problems when using a plane and how to solve them.)