Should I upgrade my WiFi router or my ISP?

So I have AT&T 50mbps DSL in my house with a Cisco Linksys router (not sure exactly what kind) and it occasionally at random stops letting me use the web even though my AT&T router has full activity (which I'm sure means my connection is good). I also have about 8 devices connected to my wireless internet (my home PS3, my Acer Timeline X AS5830, my sister's Compaq Presario C770, my parents' HP Inspiron dv7, my sister's Amazon Kindle Fire, my mom's just purchased iPad, and my dad's iPhone 4S) and I was wondering if, to handle all the devices, should I upgrade my WiFi router or my ISP internet speed?