How to know if I need a new hd or more ram?

Hi,

I have a standard late 2011 mb pro 13" 2.4 ghz i5, with 2 filled slots of 2gig ram 1333mhz and a SATA hd 500gig with 105gig free space. Running OSX 10.7.5 .

My problem is mainly when i run chrome with a various number of tabs open, everything slows down. This mainly occurs when facebook or whatsapp web + messenger is open. I have 'memory clean' telling me i have couple of megabytes ram left when it's going slow. I don't use any other heavy applications. Just chrome, pdfs, mails, word etc.

I've read already that chrome and mac aren't the best buddies and tried the newest safari / firefox my osx could handle but the same issue occurs with those browsers too.

Now i figured out that i could buy more ram but i'm not 100% sure if it will solve my issue?

Q:

How can i know if i need more ram or if i need to upgrade my hd to an ssd?

(I'm looking for the lowest cost option as i'm planning to buy the newest macbook pro, hopefully out late 2016.)

Thank you