With today's designs and production technique, the aluminum vs steel debate is a moot one. There are good points on the Cannondale Touring bikes, and, as far as I could see on the Touring list (see http://search.bikelist.org ), there are a few drawbacks.

1. I heard more problems with rear wheel failures on Cannondale touring bikes than on the Trek 520. Failures ranged from wheels that didn't stay true to numerous broken spokes to cracked rims. WHy? I can't say whether it is because more people on the touring list have a Cannondale bike, because those who have them are harder on bikes (or ride on rougher grounds, or because wheels aren't as good as those on the Trek 520. Looking at the tire width of a Conti Top Touring 700x32 once installed on my Trek 520 vs another listmember's Cannondale touring bike (I don't know which one), I suspect his rims are narrower than mine.

I've also read from one guy that there is ample clearance on front for 700 x 41 mm (real width 37 mm), but that on the rear, it was limited to 700 x 37 (real width 32 mm on his bike). The Trek 520 only has clearance for 700 x 32 front (real width 31 mm on my bike), but for more than 700 x 37 rear (real width 37 mm). So, regarding wheels and tires, the Trek has an advantage.

2. Gearing is a bit lower on the Cannondale, with 28-38-48 and a 11-34 cassette, whereas the Trek has 30-40-52 with a 11-32 cassette.
Hardcore loaded tourers and people who ride in steep environment will want lower gearing, but for everyone except those who ride on flat terrain, the Trek gearing is well suited to sporty rides whereas the Cannondale gearing gives 1 or 2 lower ratios and would suit many or most tourers as is. Advantage Cannondale, except for hardcore loaded tourers who will want to change chainrings anyway -- then it's a draw.

3. The Trek uses V-brakes and Diacompe 287 V levers. V-brakes are easy to adjust, make a clean line, and using Diacome levers mean *no* Travel Agent (which eats into cables). However, brakes have to be close to the rims, and even if the wheels are strong, it doesn't leave a place for error. Still, in 2 years, I never had brake rub. The Cannondale use cantilever brakes, which are a bit less easy to adjust, but which may be adjusted to wider gaps between brake pads and rims.
Advantage Cannondale.

4. The Trek is one of the few bikes using barend shifters. By default, the left shifter always work in friction, which means "use whatever chainrings you want". The right shifter can also be used in friction, which is good if you're fed up of adjusting the barrel screw, have a gummy setup (I do in Winter), or if you bend the derailleur in an accident. Cannondale uses STI, which lacks all the above "tolerance", and if a STI lever breaks on the road, you're stuck... or nead to replace it with a set of downtube levers you have carried around.
Advantage Trek. However, it's mostly a matter of personal preference.

5. The Trek uses a lot of Shimano parts of the drivetrain (good or bad, I'll let you judge), whereas on the Cannondale, most of these are Coda, their own private brand. How does Coda compare to Shimano, I don'T know.

6. In theory, a steel frame could be repaired on the road by any welder. However, this applies to lugged frames, because current TIG-welded steel frames either need a TIG repair, or a really skilled welder to repair with a torch. So repairing a broken TIG-welded steel frame in a small village is probably as difficult as repairing a broken aluminum frame.