If you run out of RAM, and have to use virtual memory with a normal hard disk, you are in for a world of slow hurt. Replace the HDD with a SSD, and RAM limitations aren't nearly as bad.
So yes, as the other posters have said, the CPU upgrade isn't worth it to most people, and the RAM upgrade is probably a safer choice. Unless you use programs like Handbrake that can load the CPU to 100% for extended periods of time, but don't use much RAM. In that case, the CPU upgrade is for you.
But if you don't use either CPU or RAM demanding apps very often, you could certainly do without either, and would probably never notice the difference. 8GB RAM is enough for pretty much everyone, including me, for at least a couple of years. Unless you're massively into virtualisation, of course.
The only potential problem is that if you DO start using large amounts of virtual RAM on a SSD constantly, you could burn through your write cycles, and kill your SSD within a few years. But if you use that much RAM, you would know it, and wouldn't be asking us whether RAM upgrades are worth it.