Wilwood makes nice calipers - they're a little lower on the totem pole of quality than other aluminum calipers out there (Brembo, Alcan, etc.) but work well for a lot of builds and are certainly high enough quality to work on our Broncos - and they're good looking for the high $$ builds that a lot of companies are doing now. I wouldn't worry about aluminum vs. steel calipers.
I'm with
@Yeller in that I like to have easily sourced parts and if you're sidelined with a Wilwood problem, you're probably at the mercy of when Summit or other mail order shops can get parts to you. That's fine when you're in urban areas and not in a hurry, but if you're 50 miles from Nowheresville, something available at a local parts store might be a lot more convenient.
A few other items on Wilwood - they don't use normal OEM dust seals on their calipers and I'm not a fan of that. Whether I'm picking fly poop out of the pepper on that one is open for debate, but it should be noted. Additionally, Wilwood likes to use rigid, rather than floating, calipers on their rear disc kits. This can lead to piston knock back as the rotor/axle moves (assuming you have a semi-floating, rather than full-floating) axles. It can also put additional stress on the axle seals and I've anecdotally seen some leaking axle seals on Wilwood-equipped 9" rear ends that I believe was caused by this. I've never seen an OEM that ran rigid calipers on a semi-floating rear axle. Mike Maier (noted Mustang/race car builder) talks a little bit about this in this video:
Todd Z.
