The standard practice in the RAM business is to offer a lifetime warranty, so you’re covered in any case. RAM is a commodity: If it works, different RAM won’t work “better”, it may just be more reliable.
I suggest you take the RAM you’ve already bought, install it in your mini, and see if it works. If you suspect you might be buying a newer PC3-12800 computer in the foreseeable future (like a 2012 mini, or a 2012 MacBook Pro), then it might make sense to get the faster RAM, so you have the option of moving it to the newer computer later.
But the RAM scales down to the speed of the computer - the computer doesn’t speed up to the bus speed of the RAM. RAM tends to be downward-compatible faster RAM typically will scale down to a slower bus speed, as long as it’s the same type of RAM (PC2/PC3/PC4 etc.) and fits in the same socket. The RAM bus (the pipeline connecting your RAM to your CPU) is 1333 MHz faster RAM won’t make your computer any faster, because the CPU can’t talk to the RAM any faster than it’s designed to do. PC3-12800 1600 MHz RAM is faster than PC3-10600 1333 MHz RAM, but that’s not relevant to your problem.
