SSD vs. more RAM is a tough one. Do you currently have 8 GB RAM? If you only have 4, that's definitely worth an upgrade.
Here's the things you can watch out for in your Activity Monitor to find bottlenecks (this is assuming Mavericks, which you should upgrade to since its handling of memory is vastly improved):
- in the Memory tab, check the MEMORY PRESSURE (Apple likes to SHOUT at you!) graph. If that starts climbing into yellow or red territory a lot, you want more RAM. (I'm not entirely sure what the colors indicate other than "bad". My guess is that one of the two means it needs to swap.)
- Likewise, "Swap used" should ideally never be more than 0 bytes. Depending on your work/config, that may be a pipe dream, but it's indicative of "theoretically, your machine would have been faster if you had had more RAM in this situation".
- in the Disk tab, check the Reads in & Writes out/sec and Data read & written/sec numbers. A laptop hard drive will do three figures of reads and writes in a second, and transfer maybe, on a good day, 150 MBs in a second. An average SSD can do thousands or tens of thousands of reads and writes in a second, and in your case be bound by whether your MBP has SATA II (about 250 MBs per second) or SATA III (about 500 MBs per second). The latter obviously makes a difference in sustained transfer operations, but the former is really the big gain with SSDs in everyday use; it's what people notice when they do those infamous "let's launch tons of random apps and see how fast that is with an SSD" videos — there's tons of files scattered around the disk, and the SSD can handle that with grace.
Having said all that, just combine your items two and three and get the 840 EVO for $180 and put your existing HDD in the Data Doubler.
That's assuming, anyway, that you already have 8 Gigs of RAM. Upgrading to 8 is dirt cheap and a must-have.