I think it depends on what is on offer. I got my top-up card of £20 when there was £40 worth of free games on offer like Stacking and Lead and Gold. The morning I typed it in though all there was were a couple of sub-par arcade shooters and the original Sonic the Hedgehog.
I think if you get the year-long subscription you get a great deal of content for the price. You just have to remember though once your subscription ends, you lose access to all the PSN+ conent you've downloaded until you resubscribe.