I've had two Cannondale's (T1000 and T400, same frame different components) and two Trek 520s. Both are good bikes. I like the ride better on the Cannondale. The steel frame of the Trek 520 was a plus after my aluminum frame on the Cannondale broke in Australia. Also a plus since it doesn't get dented the same way by the airlines.
I don't think you can go too wrong with either bike. If most of my riding was in the US, Western Europe, Australia, New Zealand, etc than I would favor a Cannondale T700. If I were riding in the third world, I would favor the Trek 520.
Although there are more of the *steel is real* crowd on this board, Cannondale makes a darn good touring bike. Not that Trek doesn't-- the 520 is a great bike. If you get the right deal, I'd buy either one.
As far as buying a used touring bike-- that's a tough thing to do. Because it has an alu frame, the Cannondales might be cheaper. But touring bikes go for a lot of money on EBAY nowdays.
Bikes Direct sells a Winsor touring bike for $600 new. It's seems to be a pretty good bike (I haven't seen it in person). Surly has the LHT for around $900-- another really good value. If you break down the cost of riding per mile, I think these new bikes are a better value than an old Trek you pay top dollar for on EBAY.
If you currently have a bike--- maybe you can just retrofit it to tour on? Hardtail MTBs often make a good touring bike,
I agree that there is no real functional difference between the Trek and Cannondales. Both are decent enough bikes. I also think that a good old scholl MTB makes a fabulous tour machine with relatively little modification. Stay away from E-Bay. It's been over-marketed and there are few deals to be had there anymore. Try your local CraigsList. That's where I found my '98 520 in mint condition for $550 two+ years ago. It was fully equipped and ready to go-long from the first day.
There is no way I'd pay retail for a 520 or a Cannondale, off the the shelf and stripped down. The old ridged frame MTB's can be had for $100 and are defnitely worth a look.
Kona Cinder Cone (2005) / Cannondale T700 Touring (1994)
The T700 is aluminum, not steel so that's a plus ....
I have a T700 which I used for an all purpose bike - commuting, training, touring.... It served me well but has been sitting idle for about 5 years. I will be putting it on eBay or Craig's list soon....
I bought a Cannondale T-800 simply for the parts for my LHT build....after I rode it around for a while I was actually very surprised with how well it rode for an aluminum frame. A fairly forgiving ride for an aluminum bike, and added weight will damper the ride of an aluminum bike. But steel is where its at.
I had a '93 520 and sold it last year. I kick myself every day for doing that. It was a great bike. I paid $400 for it on Ebay a few years ago so it is possible to get a deal on there, it's just not that easy. I did convert a 87 Bianchi Grizzly hard tail MTB last year to be a touring bike and it seems to be great for that purpose. Plenty of braze-ons and nice beefy steel frame. I paid $60 for it. Of course after the rebuild, I probably have close to $1000 in it, but I had a great time putting it together. Plus it's unique.
I'll leave the steel vs Alum... argument, I've ridden both many miles and they each have their own advantages.
I wanted to say if your looking at a Canondale check the tube between the head tube and BB just below the head tube. This is where they generally have the most stress, look for hairline cracks, have someone else sit on the bike and lean over the drops, look and more important feel for cracks.
I had one of the early touring bikes converted to 700c from 27" and am a very large (@300lbs) rider and have broken pretty much everything (from a brazing on a puegot to a stronglight crank arm!) but never had a problem with my Canondale!