The price of a sound card PER INPUT depends on the quality of its preamps and converters. Most modern sound cards (especially cheap ones, but not only) are total crap in terms of signal to noise ratio, sometimes so poor that they are no better than 16 bit while being technically 24. You can get clipping on instrument inputs with active pickups, or lots of noise on mic preamps. Often their line inputs go through mic preamps with a pad, which introduces noise. Sound card manufacturers don't really make either converters or preamps. And no brand can guarantee anything. The trend is to use cheaper crappier stuff even for the more expensive stuff.
That being said, it may not matter to you at all, but that depends on how you're going to use it. If you aren't going to record DI signal connecting your guitar directly to the sound card, or use its mic preamps, or do any serious recording/reamping, then just don't worry about it, choose a sound card that has stable drivers for whatever OS you're going to use and is convenient for your purposes. If you just use it to connect your Axe's line outputs to the card's line inputs, all the important conversion will be done by the Axe, so getting an expensive sound card (read - paying a lot of money for mic preamps) is just a waste.
If for whatever reason you want to go with Firewire, do check how it works with your computer - you can get tons of headache with it. If you're not sure, just avoid it.
With regard to latency, as long as the sound card has hardware monitoring, don't worry about it. Unless you plan to do recording/reamping/live processing.
