http://kbase.x10.com/wiki/Comparison_of_All_Motion_Sensors
To sum up Jim's link and give me a lot of practice using the table feature here...
(The link still looks better.)
Feature | Detection | Dawn/Dusk Detection | No Motion Off Time | Weather Resistant |
MS12A | Always | Fixed Enabled | Fixed 6 Minutes | No |
MS13A | Always or Night Only | Fixed Enabled | Variable | No |
MS14A | Always or Night Only | Fixed Enabled | Variable | Yes |
MS16A | Always or Night Only | Variable | Variable | Yes |
As you can see, the MS12A and the MS13A are best used indoors only. (If you have a kid that likes to steam up the bathroom when they shower they wouldn't be good for that indoor use either but that's another story.)
The MS12A is very limited. I'd only use it to trigger a macro unless you can live with an off signal at 6 minutes each time. All the others have programable off times. I think in powers of 2. i.e. 2 min, 4 min, 8 min, etc
For outdoor use, you should pick the MS14A or the MS16A.
I have a MS16A and it has a way to adjust it's off time with a screwdriver. I don't recall a way to vary the Dawn/Dusk Detection as the chart indicates. Personally I think I'd of been just as happy with the MS14A. (I've already lost the little rubber plug that covers the adjustment screw.)