These things are worth saving up for, that's for sure.
The full list of differences is on
the product page, but the main things are:
- Lower latency on the ADC.
- Improved SNR and THD on the inputs.
- SteadyClock FS (hence the "Pro FS" part) for better syncing with an incoming ADAT signal.
- Headphone outs are improved. I've tested them for re-amping and they worked well enough for me.
If you're saving up to upgrade to this kind of thing, the FS is almost certainly worth the extra, possibly unless a retailer has some old non-FS stock that they're willing to discount heavily.
If you're thinking of upgrading from Pro to Pro FS, the only
new feature on the FS is the K-slot for physically locking it to something. Otherwise, it does the same things but somewhat better. I'm being a little mealy-mouthed here because for some people this will take it over the threshold from nearly-there to now-it's-there, while for others the non-FS will already be so good that the improvements just offer a bit more headroom.
If you're thinking about using it for a line-level effects loop and you don't already have the Pro, you need to bear in mind that it only has two balanced line-outs. For monitoring that, you'll need to either use headphones or patch an outboard DAC via optical cable (ADAT or S/PDIF).
I've been mulling this over myself, because I got a nice mic preamp for recording bass through, and it sounds so good I'm considering getting a second one so I can run synthesised sounds through them in stereo.
The Fireface UCX II is the device I actually
wanted to replace my venerable FA-101, but it didn't hit the market until after I'd added the OctoPre to the Babyface, and it's hard for me to justify spending basically the same amount all over again to get more line outputs and ditch the MIDI breakout cable. Although, if you're in the EU (I forgot where you are, sorry) I could consider selling my Babyface at bit of a discount