RAM Upgrade or New Computer?

Backstory: I own an early 2011 Macbook Pro and it's been a great computer. I've used it pretty heavily over the last couple years, and it's still got good battery life and I don't have many complaints. However, it has seemed to be slowing down significantly over the last year, to where if I have more than about 4 tabs in Safari open it starts to beach ball and chug. I have 4GB of RAM, and looking through the forums it seems that isn't quite enough. So I have a couple options here.

1. Get a RAM upgrade. Looking on the Apple store, 8GB of RAM costs $200. That's not a horrible expense, and I've certainly taken worse hits, but I'm not sure how much that will actually upgrade the speed on my computer.

2. Get a MacBook Air. This would be much more expensive, about $600 after I trade in my own (or potentially sell it) but it seems like a good option. The 12 hour battery life would be amazing, the graphics seem greatly improved, and the SSD would be a good speed upgrade as well. Is this a good time to jump on the ship for a new Macbook or should I hold out with what I've got?