When I was preparing to climb Aconcagua, I had to make sure I had all the items on the Adventure Consultants gear list, and this meant getting quite a few things that I'd never had before. In doing the necessary research to find something that was right for me, I found the comments and reviews on various web sites to be very helpful. So here is where I return the favour. As promised, I'm going to go systematically through the categories of gear for Aconcagua to discuss what my experiences were and what I'd recommend. This post covers clothes. I make no apologies for mentioning brands and models, since in many cases their specific characteristics are important.
For the trek in, I wore shorts-convertible synthetic trekking pants and a loose-fitting ultra-lightweight Icebreaker top. I regretted not having one of the trekking shirts recommended in the gear list, as despite my best efforts to wear my buff in the Foreign-Legion style and tie a spare t-shirt around my neck I struggled to keep the sun off my neck and upper sholders. I would highly recommend getting one of these.
The gear list recommended 2 thermal tops and 2 pairs of long underwear. There's a perennial debate about synthetic vs. merino: I come down firmly on the side of merino, but that's a whole other topic. Up the mountain, I took 150 and 200-weight Icebreaker Bodyfit tops and 150 and 260 Bodyfit long underwear. If you really wanted to be minimalist, you could probably get away with just one top and bottom. I shifted to the warmer pair after arriving at camp 1 and kept them on pretty much until we got off the mountain. But they don't weigh much and it's worth having a second pair as a backup.
The gear list recommended a light fleece as a second top layer. If you can get one, a Powerstretch type fleece is highly recommended, as this material fits easily with layers below and above it. Mine was an Arcteryx Rho AR. I put this on over the 200 Icebreaker at camp 1 and kept that combination on, day and night, until we got to Plaza de Mulas on the other side of the mountain some 9 or 10 days later.
You need at least three and probably four jacket-type garments for warmth and weather protection.
You need a good Gore-Tex or equivalent jacket as the default outer layer for stopping all wind and precipitation. Mine was the Outdoor Research Furio, which worked out well. While some will prefer something nice and light, you want the jacket to be reasonably robust. It should be sized to fit comfortably over at least three layers (including the sleeves and the hood), and under pack straps. Some overlap with your pants is good, but the thigh-length jackets sometimes favoured by New Zealanders for the wet conditions here are not really appropriate for mountain climbing.
The other absolute esential is a warm down jacket. This provides your bastion of warmth while at camp on the lower mountain, and usually for at least the morning on the summit ascent. The Mountain Hardwear Sub Zero SL Hooded Jacket is perfect for Aconcagua, with just the right balance of warmth, weather resistance and packability. I was one of at least four on our expedition that had this exact jacket. You want to get the jacket, rather than the longer and more unwieldy parka model. On summit day, the down jacket needs to be worn over your Gore-Tex jacket. For this reason, and because it is cut short, if you are between sizes you should opt for the larger size.
The third jacket recommendation from Adventure Consultants was a "mid-weight jacket", but the specific examples they gave were a bit confusing. The listed options were an insulated soft shell such as the Marmot Super Hero, a 200 or 300-weight fleece or an insulated jacket like the Mountain Hardwear Compressor. These options all have rather different qualities and purposes. Based on my experience, I would strongly recommend the Compressor. What you are looking for here is a default source of warmth when your big down jacket would be too much or is stashed away. This is a jacket that goes over your trekking clothes at camp or long rest stops, and goes under your Gore-Tex jacket as a mid-layer higher up the mountain. The Compressor is about as warm as a 300-weight fleece, but much lighter and less bulky (if not very stylish -- in my green version I have been dubbed "Kermit the Frog").
The fourth jacket recommended by Adventure Consultants was a "wind shirt", such as the Marmot DriClime. This is actually what I'd call the "soft shell" category and is where I'd put the soft shell option mentioned above. This is a light, breathable and comfortable jacket that can be the outer layer in non-extreme conditions and a mid layer under the Gore-Tex jacket when wind and precpitation get out of hand. It will often have some light insulation and may have a hood. Strictly speaking, this is not obligatory, but you will be glad if you have one. I took the Arcteryx Gamma MX Hoody, and I wore this for about 60 percent of the whole trip (as well as large parts of the rest of my life). The great advantage of this jacket is its excellent fit as well as amazing appetite for abuse.
You need Gore-Tex pants as your main outer layer, and they must have a full zide zip so you can get them on and off over boots. I got the Outdoor Research Furio pants, the sibling to my jacket. These worked out well as I was fortunate enough that they fit me perfectly in both length and width (at least in the shape I was in during the climb). Some may prefer a bib or salopette, since these work with a roomier waist and the greater torso coverage provides extra warmth. This is a good option if you do lots of sking or snowboarding or are planning on doing more alpine climbing.
On Aconcagua you usually need insulated (not down) pants for summit day only. The default option here is the Mountain Hardwear Compressor, again the sibling of the jacket. These worked just fine, pack down very small and are easy to get on and off. I wore them for about the first half of summit day until it got too warm.
The gear list recommended fleece or soft shell pants. I'm not sure that anyone on the trip had fleece pants. I took a pair of MacPac Nemesis soft shell pants. I find these to be useful in intermediate conditions like climbing in Arequipa, but didn't really use them on this trip as we went pretty quickly from baking heat to very cold and snowy, so I switched straight from trekking pants to Gore-Tex. I thought I might combine the soft shell with the Gore-Tex pants for additional warmth, short of wearing the insulated pants, but never did so. If you really wanted to save space and weight, this is an item you could consider leaving out.