As someone who has used both units with the D8B, I think I can give you an educated opinion.
Both work with the D8B just fine. I preferred the 9652 but had to replace it with the 2408 because I bumped an old Mac that was PCI-only up to a newer model that was PCIe only. The 9652 would no longer work as it is PCI-only.
If your computer supports both PCI and PCIe cards, you could use either I/O unit. If your computer supports only PCI, you could still use either unit...but you'd have to order the older PCI-424 (or possibly the PC-324) interface card for the 2408. Depends on your computer model. Read this for more info on the cards:
https://www.motu.com/store_products/hardware/pcie-424The 9652 takes up two card slots and has two MIDI ports and WC connections. A major plus if your board is equipped with an Apogee WC card. Its software control is done through TotalMix. Complicated UI but incredibly detailed and feature rich.
The 2408 has no MIDI I/O. You'll need a separate MIDI I/O unit if you use an external MIDI controller. Software control is done through CueMix. Less complicated but more than adequate for the job of communication between the DAW and the D8B.
Both systems sound excellent. I was never really able to hear a discernible sonic difference between the two.
As to obtaining DIO8 cards....I'd suggest OPT8's instead. No flipped phase problems and they are strictly Lightpipe interfaces....something that both the RME and MOTU are as well.
As to how these interfaces will work I/O and transport -wise (MMC/MTC) with Sonar...that's up to how well Sonar's MMC/MTC interfaces are designed. You'll need to investigate that yourself.
So....as far as I'm concerned, it really just boils down to price vs features. 2408/PCI-424 has no MIDI but can be configured to work with PCI or PCIe card slots. 9652 has MIDI but is PCI only.
Both play(ed) extremely well with the D8B using Cubase on a Mac