Raised beds?

Having scavenged a bit of a flower bed it feels great to able to plant into the ground, I'd love to be able to plant more so I'm toying with the idea of making four raised beds out of the decking i ripped up last year, I can just about make them 16" wide (leaving just enough room to get around the flower bed) I'm thinking 3 (15") or 4 (20") boards high, any higher I think would cause to much shade, is there any benefit of 20" over 15" plant wise?
Thanks.

 

They'll bit sitting on the gravel which will be sitting on the concrete.

 

When I laid the blocks I took away most of the mortar on each join allowing water to run into the soil, with all the rain we've had recently I've only had a couple of little puddles so I'm hoping that won't be a problem.