Upgrade RAM or switch to SSD? I have a 2010 MacBook Pro (2.4Ghz) with 4GB of RAM (2x2) and a 250GB hard drive that's about 85% full. It's been running pretty sluggishly lately even for basic tasks. It is 5 years old, but a new laptop isn't really an option for a year or two. I use it primarily for web browsing (lots of tabs) and some Photoshop work.

Would I see more improvement from upgrading to 8GB of RAM or swapping out my hard drive for an SSD? Also what would the cost differential be between the two since I'm planning on only keeping this computer another year or two?

Any insight would be helpful.
Open up activity monitor and check your RAM usage. If it's high pressure (check the graph), or even swapping files, you will benefit from more RAM.

Any computer will benefit heavily from installing an SSD. Expect everything to go 10x faster (not kidding). Also, it will make you not having enough RAM less of a problem, as it will speed up your swap.

So it just depends on your budget and needs.

My personal opinion if you can only buy one? Get an SSD.