I think that if you want a tent for winter camping and mountaineering, then you should go ahead and buy one with that in mind. The fact is, such a tent will work just fine in the summerit’ll just weigh a bit more. And “convertible” tents, while pretty good, are compromise products. Moreover, winter camping in the Cascades is plenty serious, with the chance for heavy snow loads and high winds.
The Fury ($399) is a great-looking tent; well designed, sturdy and at about seven pounds not too heavy for a true winter-oriented tent. Its hooped vestibule is a particularly useful feature for winter camping, as you can store lots of stuff there or use it as a kitchen (with good ventilation, of course). The Fusion 2 ($299) will work, but has more ventilation than the Fury so will be a little colder at night. Plus, it has a stake-out vestibule, which is almost impossible to use for more than gear storage. Better for occasional winter use in fair conditions, rather than regular, serious winter use.
There are many other fine tents out there, of course. Mountain Hardwear’s Trango Assault ($395), Sierra Designs Tiros AST ($425), and Dana Designs’ Jana ($475) are all tough, winter-tested two-person tents.