Memory(RAM) upgrade on a Mid-2012 MacBook Pro

Hello Apple fans and users. My Mid-2012 MacBook Pro 13" is a bit slow. It has 2x2GB DDR3 1600MHz Ram and when I open more than 4-5 tabs in Safari it becomes really really slow. So that's what's bothering me and I have decided that I need to upgrade my RAM to more GB's. I have iStat app and it shows me that more 3/4ths of the RAM is being used when I have more than 4-5 tabs open in Safari and when my iTunes is playing a song.

So Im from the Netherlands and what I found is this: http://www.alternate.nl/Corsair/4-GB-DDR3-1333/html/product/922656?tk=7&lk=9331

It even says MAC Memory. I find this good because it is supposed to be for a MAC, but my only concern is that my memory right now is 2x2GB 1600MHz, and the one that Im currently looking at(at the link) is only 1333MHz.

Do you guys think this might be a problem or it is bad or is it going to be a NOT SO GOOD upgrade? Im not very good with computers but from what I know 1333MHz is less than 1600MHz...

But I also found this -> http://www.alternate.nl/Kingston-ValueRAM/4GB-ECC-DDR3-1600/html/product/1118143 ?event=search

and it is 1600MHz, DDR3, 4GB BUT I AM NOT Sure if it's gonna work on my laptop because it is a MAC.

So what do you guys think? Which one should I get and why? Is there a difference between 1333MHz and 1600MHz? And what is it? How am I gonna feel the difference?