Should I use a timer or motion sensor in my hot water recirculation system?
Timer: the other way is too slow in many cases.

Using this chart of water contents(gallons per foot), we see that a 3/4" pipe of 70 feet has around 1.75 gallons of water in it. Using the fact that a lavatory sink emits out either 1 or 1.5 gpm, split between the hot and cold water pipes, means that it will take 1-2 minutes for the hot water to reach the tap, making the system useless.

A timer can circulate the water relatively quickly ever few minutes and then turn off, trading some electricity for your time.  The motion sensor won't have enough time to circulate the water before you need it.

Nominal Pipe Size
(in)   Water Content (Gallons/ft)
Type K  Type L  Type M
1/8     0.0014  0.0016  0.0016
1/4     0.0039  0.0040  0.0043
3/8     0.0066  0.0075  0.0083
1/2     0.0113  0.0121  0.0132
5/8     0.0173  0.0181  0.0194
3/4     0.0226  0.0251  0.0268
1   0.0404  0.0429  0.0454


http://www.engineeringtoolbox.com/water-content-steel-copper-pipes-tubes-d_1617.html
http://www.epa.gov/WaterSense/pubs/faq_bs.html