There really isn't one boot that's best for both Shasta, a relatively trivial peak, and Aconcagua, a relatively serious one. For 14,162-foot Shasta, on most any climb not during the winter months, a good pair of leather mountaineering boots or light plastics would be fine. Examples: Salomon's excellent Mountain Expert boots ($250; www.salomonsports.com) or Koflach's Degre ($255; www.koflachusa.com). For an early summer climb, plain old leather boots would be fine. The wild card, of course, is when
you climb Shasta, April being vastly different to June.
For Aconcagua, a big, cold mountain (22,834 feet) in western Argentina, double plastics are just about mandatoryso long as you value all ten of your toes. That would include boots such as the Koflach Arctis Expe ($355) or the Scarpa Inverno ($290; www.scarpa-us.com). The Invernos are great and an amazing bargain. Very warm as is, but you can add a high-altitude liner for another $200 (you won't need this for Shasta). You'll also want full overboots for Aconcagua, something like Outdoor Research's Brooks Ranger Overboots ($136; www.orgear.com).
Now, the Invernos would work reasonably well on Shasta, provided you're on snow more than half the time. They'll be heavy, uncomfortable, and too warm for most hiking on bare rock or dirt. But they're not the worst things in the world, either. So while they'd be too much boot for Shasta (again, unless it's a cold, snowy March climb), they'd be manageable. Maybe you could buy some Invernos for the Aconcagua climb, then rent something lighter for Shasta. Or find a pair of Degres on eBay, where some guys are selling boots as I write this in sizes 9.5 and 10.
That leads me to my rhetorical question of the day: Is there anything
you can't find on eBay these days?