I use Saffire Pro 40 without problems and with good results, but I recommend Scarlett 18i20. Firewire is obsolete tech, so it's really hard to find a firewire card or even firewire cables. USB is a more persistent tech.
Focusrite has developed zero-latency monitoring by bypassing your PC (your monitor goes from the interface direct to your speaker or headphones, instead of going to your PC, back to the interface, and into your speakers or headphones). Both models has this feature. Latency only appears when you monitor along with your DAW's effects. E.g., you can't hear your voice with reverb as you are recording, but reverb can be applied after you record, in post.