I want to buy a camera for taking pictures of small models. I have been looking at the Fujifilm Finepix HS25 EXR and Canon Powershot SX510 HS, however I don't know which one will be better for my needs.
For shooting small models, the most important thing is going to be the magnification of the camera. Magnification relates to how large it is possible to make something small appear in a photo. It is related to the sensor size, the focal length and the minimum focus distance (the minimum distance at which the camera can focus).
Both cameras have roughly 1/2 inch sensors. (The Canon is actually 1/2.3inch, so it is a little bit smaller, which is slightly to the Fujis' advantage.) Both cameras have similar optical zoom (24-720 35mm equivalent), however the minimum focus distance on the Canon is far, far superior. While the Finepix requires the subject to be 1.4 feet away shooting normally on the wide side and 9.8 feet away when zoomed in all the way, the Canon can focus on something only 2 inches away on the wide side and 4.3 feet on the zoomed in side.
Using the macro feature is similar. The Canon can focus down to the lens surface where as the Finepix requires at least .4 inches. So in terms of minimum focus distance, the Canon has a clear and decisive advantage. It is offset a little bit by the fact that the Fuji is higher resolution with a slightly larger sensor, but in most cases, it doesn't appear it would be sufficient to make up for the difference in minimum focus distance.
What you basically want to looking for in the specs are the macro capabilities of the camera. A bridge camera may or may not be your best choice, here, particularly if you want to handhold the camera for the shots. Bridge cameras (the ones that look like dSLRs and are typically marketed by how big the zoom factor is), tend to trade off reach for low light capability. Get the bridge camera if you also plan on doing a lot of outdoor daylight shooting. Otherwise, you might want to look at advanced/enthusiast compacts (if budget permits), such as the Canon Powershot G and S, or Panasonic LX lines.
For shooting miniatures, if you want to do product photography-type "glamour" shots, the trick is mostly going to be the lighting, as well as the macro capability. So, what you're probably looking at in specs of any camera are going to be the minimum focus distance (i.e., how close to the object the camera can focus), and whether or not the camera has a flash hotshoe. I'd also advocate looking for a camera that has full Manual (M) mode for exposure setting control, and a RAW capability.
And you'll probably also want to look into getting a tripod (because the more you magnify the image, the bigger the shake blur is going to appear), whether or not the camera can use a cable release, and off-camera lighting gear.