I want to track a few hundred, maybe a few thousand people in real time.
Let's say that the hardware aspects are sorted out and I can get the data into a database.
Now, I want to get it out and show it, in real-time.
Weeeell ... "real-enough" time. Let's say that I want to draw a floorplan of a building and plot everyone every 1 to 5 seconds.
(I might want to show only certain "kinds" of people at the click of a button; I will need datamining, etc, but let's stick with the worse case scenario).
I am comfortable enough with PHP, though not this sort of thing. I personally would be happier with a windows app coded in Delphi, but the trend seems to be to make everything browser based.
So, the question, I guess is whether a browser can handle this and whether there are compelling arguments for a windows-based or browser-based solution.
If browser-based can handle this (displaying a few thousand data-points a second), and there are no overwhelming arguments for windows then I guess I will go for browser-based and learn a few new tricks. The obvious advantage being that I could also re-use a large part of my code for (vehicle) tracking on Google maps.
Most of your work will probably be done in your spatially enabled relational database. For example, PostGIS can select data points within a bounding box or more sophisticated spatial predicates (ST_Contains, ST_Crosses, ST_Intersects, ST_Touches, ...) as well as the usual SQL joins and WHERE conditions. Spatial selects should use a spatial index to speed things up.
If this is the case, your app will largely be a presentation layer. In this case, use whatever will be easiest for you. The advantage of browser-based is that it is cross-platform client-server by default, but this might not matter to you.
With respect to render speed, it really depends on how you are planning to render your map. There are speed tests available for Google Maps. However, I suspect that if you are planning on a google maps type interface several thousand points are going to turn into a blur of pins. Do you have an interface mockup?