This is a lab-obtained value that conveys the insulating properties of the sleeping mat. The higher the number, the better the insulation. Since the measurement is made about the resistance of a sleeping mat to heat transfer, the larger the R-value, the better it is at retaining heat, i.e., better for winter.
With the lack of standards and labelling rules there is not a great deal of R-value consistency between brands. So take the reported R-values with a grain of salt and think though the style and materials used before assuming one mat is warmer then another.

Of course, every person is different and their tolerance to cold will vary with the way they sleep and the sleeping bag they use. The R-values are what the mat can do at its extreme and not loaded with a person lying on it (this reduces insulation). Select a mat with an R-value that is higher than you expect to have a comfortable night.

Sleeping mats vary in size by their thickness, width and length. Generally they are about 180 cm long, and about 50 cm wide. Thicknesses varies greatly, from about 8 mm for closed cell foam mats, to about 30 mm for self-inflating mats to a very comfortable 50 mm or more for inflatable mats. Some bushwalkers prefer three-quarter or half sized lengths to save on weight. Bushwalkers may use clothes or other material to provide insulation under their feet with shorter mats, but full-length mats tend to be more comfortable. As a general rule, the thicker the mat, the more comfortable they are and the lower the likelihood of pressure points. For some people, particularly people with some disabilities, a double width sleeping mat can make the night much more comfortable and mean they are less likely to come off the mat during the night.

Some sleeping pads are rectangular, while others are mummy-shaped, following the body’s natural shape. For instance, the Therm-a-Rest™ Women’s ProLite. This has the advantage to save on weight due to less material.

Some sleeping mat surfaces are really slippery, causing your sleeping bag to slide off easily, particularly if you move around a lot in your sleep. Some mats will move on the tent floor. Textured surfaces tend to have more grip and can provide a more comfortable sleep. Too much grip may make it hard to move around. Some sleeping bags have pockets to hold the sleeping mat. If you have this design, then you tend to want more slippy mats to make it easier to get in and out of the sleeping bags pocket. Also think about the noise generated by the material; in a quiet hut or campsite you can drive yourself and others crazy each time you roll over with noisy material.

The two factors that affect inflation time are the size of the mat and the type of valve. Some valves transport high airflow volumes, meaning that the mat inflates quickly. It’s far more enjoyable to inflate these mats after a long day on the track! And of course, a three-quarter mat will be much faster to inflate than a similar full-size model. Some mats are self-inflating, while others need to be inflated by mouth. Orally inflated mats tend to be slow and over time can have mould build up inside the mat. To avoid mould growth, use a dry bag pump, as these not only make inflating mats much faster and easier but the bags can also be used to keep gear dry.

Side rails are railings on the side of a mat that make it harder for the user to roll off the mat during sleep. These are not common and I am not sure how much they help.

If this was an English lesson I would say a baffle is something that restrains or regulates, but since this a bushwalking article I will say they are fabric that joins walls and hence create chambers to minimise airflow and the loss of heat through convection. Baffles give a sleeping mat, sleeping bag or down jacket better insulation. Generally the more baffles the better the insulation, but they also add weight due the extra material. Also, baffle design effects the comfort on the mat. This is probably more of a personal thing, so worth trying out a few designs to see how they work for you.

Insulation has really been covered above. R-value is really a measure of the overall insulation performance of the mat. Baffles play a role in this, but inflatable mats can be partly filled with down or a synthetic insulator as well, helping reduce heat loss through convection and conduction. Also, some will have reflective material to help reduce heat loss through radiation. The value of these vary greatly depends on the overall design and interplay with each type of insulation.