It depends. If you intend to use the modular for a lot of live work, the ES-8 makes far more sense as it eliminates extra gear that you'll have to deal with if you went with an outboard interface. But in a studio environment, I'm finding that using an "old, obsolete" interface like my MOTU 828 mkii is better as it's not specifically "assigned" to one piece of equipment. For example, the 828 mkii I use needs to be able to feed or use outputs from:
A 22 module Digisound 80
The 180-space AE modular (via a Soundmachines Nanobridge)
A Landscape Stereo Field
Bastl bitRanger and Softpop
A B.2600
A Keystep and Keystep Pro
Two Beatstep Pros
Korg SQ-1
Electro-Harmonix 8-step
Koma Field Kit and Field Kit FX
Two Korg MS-20 minis
Moog Subharmonicon
4-channel Symetrix gate
4-channel Frederick Haer window comparator
Arturia Microfreak
...and probably anything else in the studio that uses typical analog control signals via the network of tie lines in here. So, with an external converter, it's simple to just use a 1/4" patchbay to route the I/O from the back to the front, then patching to anything is a snap!
And don't EVEN ask me about how the MIDI routing in here works!