When looking for a wide angle lens you should consider a few very important things:

• Focal length
• Image Quality
• Maximum Aperture
• Focus modes (AF/MF)
• Filter compatibility
• Feature set (FTM, IS, USM, etc.)
• Mount
• Weight, size, cost, etc.
• Intended usage
• Distortion & Projection
• Flare resistance

Most of the above is not uniquely important to wide angle lenses, but the items in bold I would suggest are of particular interest to wide angle lenses.

Focal Length

Focal length is of utmost importance when considering a wide angle lens. If you want to shoot indoors and you can't simply back up your body, you may require a 17mm lens for a shot. Yes you can stitch images together if desired or necessary but you may want to buy the focal length that best fits your desired output to begin with(See: Do you still need an ultra wide angle if you can now stitch images?). There are many online visualization tools to determine what an image looks like at various focal lengths, I would encourage you to check them out first - but ultimately the best way to determine the right focal length is with some first hand experience using them. Finally, as you go wider distortion will certainly bear its ugly head. More expensive designs will help the situation but it is safe to say that wider lenses will typically experience more distortion. See this question for more information: How much does 1 mm get you in a wide-angle lens?

Aperture

The practical difference between f/2.8 and f/1.4 is for most situations identical with a wide angle lens as with any other focal length. The advantage of f/1.4 vs f/2.8 is that you will be able to shoot in significantly darker situations, or in the same situation with either a faster shutter speed or lower ISO value. It also gives you greater control over depth of field and the ability to create shallow depth of field images even at wide focal lengths. These are very wide apertures for any wide angle lens. Most wide angle lenses are actually quite a bit more narrow in the aperture area. See this question for more information on why: Why do wide angle prime lenses have relatively small apertures?

Filters

Filters are somewhat unique to wide angle lenses as some very wide lenses(especially fisheye) do not have the ability to use screen on filters. If you want to use filters(ND, Polarizer, etc.) you have to take this into consideration. You may have to either buy a new filter system or not use one at all depending on what you desire. Further you have to consider the impact of using a polarizer with a wide angle, as it can produce poor results.

Overall

Overall, the two lenses that you described are most importantly different because of their focal lengths. I personally would not be using them for the same types of shots as the field of view is so different. If you demand very wide apertures(astrophography, portraits indoor, etc.) you may also benefit from the wider aperture but that is a personal decision.

Additional Resources:

• Do I really need a wide-angle zoom lens?
• Is image stabilization a necessary feature for wide angle lenses?
• What to look for when testing a wide angle lens before buying?
• How do I compare two prime lenses of different focal lengths?
• What to look for in a wide angle lens?
• What filters will work with an ultra wide angle lens (for landscape)?
• Is it wise to use polarizing filters with wide-angle lenses?