Outer garment selection is a lot simpler if you don't perspire, aren't concerned about durability, are just a walk around town tourist, and have a generous budget. For their rest, outer shell garment selection is an exercise in compromise.
REI's rainwear selection advice is a good overview of the decision factors:
http://www.rei.com/learn/expert-advice/rainwear.html
Main Considerations
Function: Will it keep me dry? When new, rainwear touted as
"waterproof" is rain-worthy. Over time, however, rainwear fabric
requires care to revive its water-shedding ability. (Such maintenance
is often overlooked.) What about soft shells? Most are
"water-resistant," meaning they can withstand light precipitation but
not a drenching rain.
Comfort: Will I swelter inside it? While waterproof/breathable fabric
technologies can make rainwear comfortable to wear during vigorous
activity, so can other factors: use of vents, individual metabolisms,
humidity, temperature, the clothing worn underneath rainwear, and
more.
Durability: Can it handle hard use? Face fabrics (nylon or polyester,
usually) come in different weights, or denier, a spec not always
provided by manufacturers. Though 70-denier (70D) is a common weight,
outerwear fabrics can range from 15D (wispy) to 450D (burly). Aim for
midweight (70D, often found in 3-layer designs) or higher if you like
to climb, scramble or bushwhack off-trail. Sticking to trails? Save
ounces by choosing a lightweight design.
Appearance: How do I look? Rainwear designed for the trail aims for
low weight, easy packability and pocket positioning that won't
interfere with backpack straps or a climbing harness. Rainwear for
travel or around-town use is styled with a fuller cut, more pockets
and less fussiness over weight.
Price: Why such disparity? High-end rainwear usually features a
waterproof/breathable laminate, meticulous seam-taping and exceptional
detailing, right down to tiny protective awnings over zipper heads.
Lower-cost rainwear is cut looser and often uses waterproof/breathable
coatings (generally less breathable than laminates). Still, coated
rainwear is quite adequate for moderate activities, and it's light and
easy to pack.
Tip: Use your past experiences to help you shop. Are you seeking
something more breathable? More durable? Lighter? Something exactly
the same? Consider the typical intensity level of your activities and
your personal metabolism, too.