Generally, more RAM has had more effect on typical desktop performance than has clock speed, but obviously it depends on how much of each is being compared. I've usually found that a 2x in clock speed feels like a decent improvement; I wouldn't expect the less than 40% increase you're describing would make a huge difference, unless you expect to be running long, compute-bound jobs, untypical of most desktop users. However, 4GB of RAM is already generous for most users' job mix, the most likely exception to this being keeping one or a couple of VMs loaded.
As a point of reference, I'm sitting in front of an older 24" iMac, 2.16 GHz dual core, 3 GB of RAM. The only time I see any slowdown is if I drag a Windows app with a large window (in a VM) quickly across its own desktop. There's enough memory for two VMs (maybe more - I haven't tried) and OS-X never gets particularly cranky. My VM's don't sit and crunch numbers though; they mostly only launch and run some interactive app or other.
Another data point: With a Youtube video running in Firefox in each of OS-X and a WinXP VM, simultaneously, there's no slow down of audio or video in either context.
Bottom line: your expected job mix will determine which of those choice will be most effective for you. For most people and most job mixes, having two hands and one attention span will be limit their ability to use more or RAM than you describe.