I don't have any specific recommendations for you as far as specific make/model, because I feel that control surfaces can be a very personal preference decision as far as overall size, control layout, number and types of controls, etc.
For me, the number of physical sliders and control knobs per slider are fairly important; I'm not a huge fan, for example, of having 24 channels of control thrown together into only 8 sliders and having to switch between 3 different "banks" of channels that the sliders can control. Also, having enough knobs per channel to handle basic channel mixing functions such as trim level and pan, including maybe an extra programmable one would be nice to have. Also, buttons programmable for Mute and Solo and maybe an extra one for buss assign or something like that would be great.
But then you gotta balance all that against your budget as well, and there is where a great deal of personal preference has to come in; which features you are willing or able to pay for is going to be a big part of the equation.
As far as the FW vs. USB, I personally wouldn't worry about that too much unless your USB bus on your PC is already pretty heavily loaded. But on the control surface end, the control surface is really little more than a complicated mouse, it doesn't really require much bandwidth at all. I find that usually the manufacturers take this into consideration, and I have not found a USB device that was really "underdesigned" by using USB tn a way where the interface itself really causes a bottleneck. Aggain, maybe if you have really loaded down your USB bus alredy with external drives, USB keyboard and mouse, audio interface, USB waffle maker, etc., then maybe there might be more of a consideration.
G.