Firstly let me explain these two intruder detectors in a bit more detail.

1. Acitve Infrared (IR) Beam
   This detector consists of two parts, the transmitter and the receiver. The transmitter emits a beam of infrared (IR) light and the receiver picks up this beam. When the beam is interrupted, the receiver alerts its alarm control panel by opening a relay. Most beams used in security applications use multiple IR beams in order to reduce false alarms and to increase the area covered.
   Pros 
   Can cover a long range. Some beams extend to 150 metres.
   Narrow detection area, so eliminating areas like busy thoroughfares is simple.             
   Cons
   Installation is tricky because the beam must align with the receiver.
   Both the receiver and transmitter need power, increasing the amount of wiring required.
   Prone to false alarms from leaves or birds etc interrupting the beam.
   Easy to defeat if you know where they are by ducking underneath the beam.
   Not pet friendly unless you mount them above the height of pets.
2. Passive Infrared (PIR)
   A PIR is wholly enclosed in one part. It consists of a passive IR receiver that measures ambient temperature. If this temperature changes rapidly, like when a person walks across its field of view, an alarm condition is sent to the controller. Outdoor PIR are weatherproof and their lenses can accommodate the harsh ultraviolet rays of the sun. Like the beams they usually have more than one detector, both must trigger simultaneously, to avoid false alarms.
   Pros
   Only one unit to install.
   No alignment issues, just mount it and you’re done.
   Wide detection area so very difficult to defeat.
   Pet friendly.
   Wireless option available.             
   Cons
   Short detection range, 12 metres is about maximum.

So with a bit more information you are better equipped to choose your detection system. Usually a combination of both will work for maximum protection. But in our opinion, if you were going to choose one detection method, PIR beats beams.