Either way won't be much of a difference in speed.
I have 2gb in my Imac and have quite a few apps open and it is quite fast. Don't often hit the limit. Even have 4gb sitting here in its packaging and haven't seen a need to install it yet.
Having a SSD improves the biggest bottleneck and in the MBA that is taken care of no matter which model you choose.
When your programs launch in a second or two from the SSD then it doesn't matter much if it wasn't in memory to begin with and has to be paged into memory from the SSD.
It really depends on what you are doing. For example, If you run any virtualization software at all you
will hit the limit. Also, on Mac OS X it never hurts to have more RAM; the OS uses inactive memory as a disk cache (which is why launching applications the 2nd time is always faster than the first, among other things).