I have went through both specs (not thoroughly though).
AMBA and Wishbone both give me good impression. Let state some pros and cons.
+Wishbone: Easy to interface to OPB (Xilinx-Microblaze SoCs)
+Wishbone: Easier than AMBA in general.
+AMBA: Single-cycle master handover (not sure if this is possible in Wishbone).
+AMBA: Has more sophisticated transactions in addition to those of Wishbone.
+AMBA+Wishbone: Reference designs can be found for both in the web.
+Wishbone: Rather a collection of bus architectures not a single bus arch.
+AMBA: Maybe timing is given in more detail.
+/-AMBA: Less decisions to the system integrator
+Wishbone: More flexibility+decisions to the system integrator
It is not an easy decision. Generally smaller SoCs could use Wishbone, and ARM-based SoCs take fond of AMBA. But that's about it. Mem. bandwidth, num. masters-slaves, and other issues are important as well.
the_penetrator©