There are differences in spun poly fabrics and acrylic fabrics -- brief description:

Spun Polyester

Unlike solution dyed acrylic fabrics, spun polyester is woven first and then screen printed which allows for a wider range of colors and patterns. While this fabric is usually less expensive it tends to be stiffer and rougher to the touch. Spun polyester also has a shorter UV ray resistant lifespan than most solution dyed acrylic materials, so we don't recommend using it on high-traffic upholstered pieces like outdoor cushions, instead save this material for more temporary applications like throw pillows. If you do use this on your cushions we would suggest covering or bringing them inside when not in use.

Solution Dyed Acrylic Fabrics

Considered the workhorse of outdoor fabrics, solution dyed acrylic fabrics tend to be priced on the higher end of the spectrum but are well worth the additional cost. They are made from a liquid acrylic solution that is mixed with the dye and then spun into a yarn, which is then woven into a fabric. These products are most like indoor fabrics and feel soft to the touch. The biggest benefit to using a 100% solution dyed acrylic fabric is that it is naturally water resistant and has the ability to hold up to constant UV light exposure without rapidly breaking down.  One negative about this type of material is that it is often limited in color and pattern due to the manufacturing process, but is a great options for larger upholstered pieces.