DDR3 memory vs SSD??

I currently own an older MacBook Pro with 4 GB DDR3 memory. I am considering the purchase of a MacBook Air with 256MB SSD memory. I really don't understand the difference. Is 256 MB SSD at all comparable to 4GB DDR3 in terms of speed and multiple app usage? I use PhotoShop quite often but other than that, I just check mail and a some web use.