SSD or upgrade RAM (slow hard drive)

I have a late 2012 MacBook Pro (13" baseline model), and it has served me well, but in the last few months it has been very slow. I'm pretty sure it's the hard drive that's slow, because I have tried to reinstall OS X, but booting, loading applications, etc. is still slow.

The first thing I thought of was getting an SSD (probably using one of those optical drive replacement kits) and I figured this would help to speed up booting and loading applications.

But the other problem is, since it only has 4GB of RAM, I often use a lot of it. I like to have safari open a lot of the time and it uses a lot of memory. However, the "memory pressure" in activity monitor is usually OK. Also, my thinking is that with an SSD, the RAM might not matter so much since the swap would be faster.

Would the speed of the SSD make up for the lack of RAM? What would you suggest?