Two patients are brought to the emergency room. One has cut his finger with a kn

The patient that stepped on the nail is at greater risk for infection.
1)  It is puncture wound, and efforts to cleanse, even through irrigation, which is generally not done aggressively, will be in effective. (Though an IV catheter can be advanced through the wound track and some irrigation can be done, two catheters are best, one for input one for output, it works but in modern medicine they just get antibiotics and a tetanus)  Bacteria, and simple debris can be trapped within the wound. 
2)  It is a puncture wound so when it does begin to get infected, it will hold puss in, rather than draining out.  Wicking the wound or an iodoform drain is generally ineffective, the patient does not tolerate it, and the would is to narrow to gain appropriate access.
3) The puncture wound is on the foot, an area generally associated with being unclean, moist damp, a good place for bacteria to grow, and most patients will not alter their lifestyle for a wound like this.
The foot is associated with poor circulation, there is a reason the diabetic foot ulcers are on the foot.  The problem is often getting well oxygenated blood to the capillary beds, which might sound odd but with diabetics it is a vasculat issue.  The problem can be getting the blood out of the legs, either way it does not circulate through as well as we would like, the extreme example of getting the blood out is patients with venous stasis ulcers.  Then simply bearing weight on the wound, the mechanical compression, will prevent blood flow.  Patients are unlikely to actually stay off the foot until the end of the second or third day when the pain from infection begins to set in. 

As far as the finger lac,
Wound edges are smooth, meaning less exposed surface area that was compromised, and exposed to contamination.  The wound can be easily opened up for visual inspection and irrigation of lower structures to ensure no joint, tendon, or bone involvement.   
Circulation is better than a foot, and unless there is a reason for vascular compromise on the finger you could even say it is good. 
The area is easier to keep clean.
Better patient compliance.

The puncture wound has a greater chance of becoming infected than the knife cut because the cut from the knife will bleed freely, washing away many of the bacteria from the wound site. In a puncture wound, bacteria can be forced beneath the surface of the skin and past the skin's protective barriers, thus increasing the possibility of infection.