No expert on this. Some comments from personal experience. On the OMAP-L137, ROM bootloader loads and runs the DSP bootloader. The DSP bootloader loads and starts the ARM bootloader. So the DSP can load and start the ARM code. The OMAP-L137 and OMAP-L138 have DSP and ARM "coupled" together to the same level. Just one is DSP boot and other is ARM boot.
I would say choose OMAP-L138 over the OMAP-L137 if only because the OMAP-L138 is slightly better supported in SW by TI. There is a version of the bare-metal StarterWare code for the OMAP-L138. TI has not offered much new or updated much for the OMAP-L137 since it first came out. From an HW perspective, the OMAP-L137 can be quite difficult is getting the peripherals you want multiplexed out to pins. The OMAP-L138 is more flexible. Still not as good as Atmel or ST's pinmux flexibility but better.