Doepfer has a couple of options here that boil down to combinations of these two questions:
A) Do you need a USB connection or is a regular MIDI connector enough?
B) Do you need MIDI Clock?
A built-in USB-MIDI interface is a little more flexible than the traditional DIN-type MIDI connectors since you won’t need an extra adapter when controlling the system from a computer. All interfaces have regular MIDI connectors (the 5-pin DIN type) so if you want to connect a keyboard or an old sequencer or something you should be set either way. In my case I already have a USB-MIDI dongle so I think I can save a few bucks by skipping the USB option.
MIDI Clock is a bit trickier. The primary use is to synchronise any sequencers you have in your modular system with external sources. MIDI Clock could be potentially useful even if you don’t have a sequencer module though. It seems nice to have a clock source available in the system, for example to synchronise an LFO with the song tempo. Unfortunately the price difference between a simple MIDI-CV (MIDI-CV/Gate) interface and one that can also handle MIDI Clock (a MIDI-CV/Gate/Sync) is pretty substantial.
I haven’t really decided yet. I imagine that my needs are pretty basic so my current pick is a Doepfer A-190-2 MIDI-CV/Gate interface. It is a cheap, no-frills interface. However, Doepfer has the A-190-4 MIDI-CV/Gate/Sync interface coming out this spring and I may end up feeling that I can’t live without MIDI Clock. It’s my first module and the Euro-quicksand is already pulling me in!