Is there an informed comparison between sending MIDI over USB versus directly from MIDI interface on an expansion card - would a MIDI interface on a sound card provide some useful system enhancement over using a plain USB 2.0 connection (perhaps to a MIDI converter)?
The USB 2.0 standard does not allow for "low speed" or "full speed" devices to be polled more than 1,000 times/second; "high speed" devices may be polled up to 8,000 times/second, but require fancier electronics. A MIDI interface takes 320 microseconds (0.32 milliseconds) to send each byte of data; if multiple notes are pressed simultaneously, two bytes will have to be sent for each note after the first, so simultaneous notes will be played at least 0.64 milliseconds apart.
Having MIDI timings rounded off to the nearest millisecond would represent a loss of timing precision, but it's only slightly worse than the loss of timing precision imposed by the MIDI standard itself. An interface which communicated as a "high speed" device could have a 1/8 millisecond polling interval, which would cause only minimal loss of timing accuracy; I don't know how many interfaces do so, however.
older usb midi throughput would lag a little in SOME drivers due to usb sending "blocks" of data. Now adays, those problems are gone.
In addition, some usb midi can cause audio interference (my Yamaha CP-33 suffers from this) so I have to use the midi port and a midi-usb interface to get around that. Very few usb midi setups suffer from this, though. Midi has an opto-isolator in the hardware mix that THIS midi usb setup obviously lacks.
In -general-, usb midi is way nicer than midi => midi/usb interface => pc
And, of course, if you want to connect two KEYBOARDS together, your only option is midi as usb midi only connects you to a computer (in general)
Only way to know for sure is google it (you MAY find out something, but probably won't) then hook em up and TEST.
And finding a card that'll do midi i/o would be great, but probably spendy and will only be found with a TON of other audio hardware on it that you may not need.
I guess when using equipment that only have MIDI-output, it comes very handy. MIDI is a well-established technology, with a low chance of failure (while with usb, I encountered otherwise...)
Most modern devices with some sort of relevant output will have usb, but I think it's worth looking for the soundcard which does support MIDI i/o.
(after all, it also gives you another port. could be handy when connecting multiple devices.)
MIDI over USB is just fine. Apart from anything else, your "sound card" that includes a MIDI port is almost certainly an external USB-connected device these days!