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?
I had installed on my MBP mid 2010 a Samsung SSS 830 250GB. 
With SSD you will solve all the problems caused by HD latency and speed (in my case speed up is around 10x). 

Of course you also take advantages on speed when you use swap memory with an SSD. It's like when you write and read something from a disk.

In summary, if you want a huge speed up go for an SSD.
I think the best SSD) now is the Samsung 850 Pro 256GB  (storage / price. 
RAM upgrade is required only if you do intensive computation / gaming / image or video editing.