As I am in a similar situation (looking to record from an Expression Si 3), I have been watching this thread for a while, and information has evolved a little over that time.
Multi-Digital I/O -- will apparently record 32 channels, so not the limitation as supposed earlier in the thread. This seems to be around the US$400 mark. If you use a PC then you can use the USB port into your DAW using the soundcraft driver (not for Mac though ... for reasons unclear). Cheap ... but no option to concurrently run a stagebox (ie no second slot)
Dante -- was originally my first choice, but as supposed earlier, this is much more expensive (the card is approximately 4x the Multi-Digital card, at around the US$1600 mark, from what I have read, or RRP AU$2300 here in Australia) Dante enabled stageboxes seem to be few, and quite high end (read: expensive). It is unclear to me how well they will integrate, particularly as the card does not seem to be shipping yet.
So from my point of view this brings me back to MADI, which seems to be the way Soundcraft are heading (given the architecture of their stageboxes). The stageboxes also come with the card, which (on the face of it) makes it more economical than (say) Dante.
Again, from reading around, it does NOT seem that it is as simple as just connecting everything together with some Cat5, and routing the appropriate inputs / outputs and 'away you go'. There is even a suggestion that you can not do both simultaneously, without the use of an auxiliary card IN the stagebox.
Has anyone actually run a multitrack recording from an Expression console into a PC/Mac, whilst using a digital stagebox via MADI.
What hardware was used? Cat5 / Optical? How many tracks? etc.
(apologies for the long first post)