So: random money has dropped out of the sky upon me such that I am unexpectedly able to consider the purchase of a new-to-me bike this year that I had been planning for next season. Now I have a fantastic problem: what do I buy? At this point, there are two used bikes on the open market that have caught my eye: a 2011 Kona Zing Deluxe with full Ultegra for $1300, and a 2011 Argon18 Platinum with full Dura-Ace - but I've only just started looking! Should I pounce on one of these, or bide my time and wait to see what else comes up for sale this summer?

Extra difficulty: the Zing is a pretty awful paint scheme - bright reflective green on black - and I've always had a soft spot in my heart for both the styling and the Canadian-ness of Argon18s. I can afford either, but I feel like I'd have to find a way to justify to myself an extra $500, beyond just "it was prettier". Is the Dura-Ace upgrade from Ultegra worth $500 all by itself? I appeal to the wisdom of the many on this one.

What do you already have, and what niche are you looking to fill?

I'm currently riding a 2002 steel frame KHS Flite 500 which at some point prior to my ownership of it was upgraded to a Campy Xenon grouppo. I'll commute 40km per day on that bike when it's sunny, with longer rides up to 150km semi-regularly. I don't race, but I wouldn't want to rule that out at some point - I'm still pretty new to cycling semi-seriously. I would like a ride that's comfortable enough for gran fondo distances, but aggressive enough to let me ride as fast as I want - I've learned from having to commute on a hybrid when it's raining that I love riding best when I'm going fast. Thoughts?

Why not buy new? $1800-2000 buys a nice bike. You get exactly what fits you, you get warranty coverage, and you typically get free tuneups. And everything works crisply.

Conversely, a two year old bike, assuming its been ridden a fair amount is going to be showing some wear. To get it to perform close to new, you're likely looking at a new chain and cassette. You'll also likely spend money for a new stem to get the bike to fit you, and you'l likely buy a new seat to meet your preference (something that can be swapped out on a new bike at little to no cost usually).
Also you may be looking at new tires and brake pads. So not unreasonable to add $200 or more to the cost of the used bike, as fitting/refurbishing costs.

As an example a new Canondale Supersix 5 (with 105 components) has an MRSP of $2399, and our local dealer has a 20% off sale right now. So you could have a brand new Cdale, with warranty, fitted to you, for less than you'd likely end up spending in total for the Argon.

IMHO, used bikes make a lot of sense with a budget under $750-1000 because it's hard to get a new bike without significant compromises. With a budget of $1500 or more you can get a very nice new bike, and it becomes more questionable to go used, just to get a slightly higher line bike.

I've always liked how those Argon bikes look, but i have no idea how they ride.

That's a very compelling point, merlin. I guess I kind of assume that I'll get an overall higher end bike if I buy a used recent model, just given depreciation; that's how i buy cars and other big ticket items I'm able to do my own maintenance on. But fit is a big deal here, and that's somewhat unique to bikes... Thanks for the input!

For the sake of completing this thread, quiet though it was: I ended up buying the Argon at a negotiated price of $1500, and it has so far been an absolute delight to ride. Thanks to everyone who offered advice.