SSD is several orders of magnitude slower than RAM.
stepping up to 8gb of RAM is much cheaper than an SSD of 128gb or more.
SSD will help load things into memory faster, but if you are actually doing any sort of processing on large amounts of data, having enough ram will be far faster than tryign to swap to SSD.
it really depends on your workload. if your workload doesn't involve large data sets, SSD will be faster. if it does, you'll see more benefit from more ram.
Does having more RAM make your computer faster even when you are not multitasking?