I've driven both the Century and the Columbia. There's different options you can get with them... and I'll touch on that in a moment.
As was explained above in another reply, the only major/main difference between the 2 trucks is the front clip, look at the headlight set-up of both and you'll see what we mean. Personally, I like the Columbia's front clip a bit better, it's more streamlined.
The sleepers have upper cabinets in some, and nets in others. I like the cabinet doors better. Glue some kind of fabric (if you want) to the inside of the doors and you'll never hear cans rattling. Why not the nets or curtains on cheap rods? because the cabinet doors just give the sleeper a cleaner look and hide what could be messy and cluttered cabinets a whole bunch better. Not to mention that stuff can pop outa those curtains and nets easier than those cabinet doors.
You can also get them with a lower sleeper window as an option, but it's rare to find them that way.
Some have a sidebox door on certain sides.
I like the FLD's ride better than the Columbia/Century though, more solid and less bouncy.
While I've never driven one, the Volvo's have a really nice dash layout and interior layout. The difference between the 2 or 3 big main Volvo's is that if it has 2 windows on the sleeper, it's got the biiiig sleeper, otherwise it's got the 61" sleeper with very little storage from what I've seen and heard by others who've driven them.
The sleepers on a mid-roof to me, are for those who are more regional. They have much less interior storage, if you're tall and want to stand up to be able to change clothes... Do Not Get a Flat-Top. Flat-Tops are for those that just want to lie down to sleep once ina while.
While you're considering trucks, talk to different drivers if you're on the road... talk to guys with flat-top, mid-roofs and condo's and ask them why they chose them... and/or what they like and don't like. If you want to cook, change clothes easily, and intend to be out for more than 2 to 3 days, do not get a Flat-Top. If you're going to be out for more than a week or 2... consider a full condo sleeper, a tall sleeper.
I loved the FLD's and Columbia/Century's under bunk storage. That storage does not exist in the Volvo's, because their bunks are set up entirely different. I was even able to keep alot of clothes in the cabinets, along with food in the Freightliner.
I'm not being partial to them, but Freightliners are the only trucks I've ever driven "extensively" over the road. I've driven some International's, but only the Eagle Pro Sleepers, and I wasn't impressed with them. Their dash layout and overall truck set-up didn't appeal to me.
You have to consider how much stuff you'll be carrying: clothes, food, microwave, TV, DVD/VCR player are standard items for most OTR drivers who stay out for more than a week or 2. Throw tools and any other hardware into the equation, and space gets limited quickly. Consider a double bunk, that way you have a place to put extra clothes and dirty laundry.
Bigger is better you ask? On "average" is the 70-72" sleeper,
below that are the 60", and 42". and the 36" "cigerette box".
The big Volvo has a 77" sleeper that looks and feels huge.
But extra lights and chrome on a Volvo is like lipstick on a pig, it just don't do anything for it.
13 speeds aren't "Needed" really, unless you're pullin' mountains and heavy loads constantly... or you just like to shift.
9 or 10 speeds, or the Super 10 speed (known to some as the lazy mans tranny) are standard amongst most trucks. 9 or 10 speeds will suit you just fine in expediting.
More and more one is beginning to see the automated manuel tranny... which is a souped up manuel tranny. you can shift manuelly or put it in automatic and let it do all the work for you... you only use the clutch for 1st and reverse. Bare in mind that repair costs for those tend to be a bit more because they're a bit more involved in their electronics and gearing.
NiteCreacher has a nice Coronado as seen above in his reply. The Coronado is a nice sharp big truck, it gives you the feel of the FLD without the boxiness of the FLD. The Coronado also has standard built-in tool boxes, something not found on any other trucks that I know of. Sounds to me that you're lookin' for a smooth truck, without the large breathers that say a KW W900 or Freightliner XL might have on it's sides... you can get a decent used truck for less than 30K if you're keen and look around.
I recommend you check out the dealers websites on the top left of the home page here on EO, those dealers are highly recommended for good reason, we've (those of us here on EO that drive or own trucks) used them and have been pleased with them. They provide good service and don't sell you anything you don't need.
Especially highly recommended is Freightliner of Knoxville, TN.
After you check out whats online that you like... go to a dealer with a wide variety of trucks... and get in them, and SIT in them.
Get in the drivers seat, and look around like you're driving it.
Does it feel right? Do you think it's "YOU"?
Get up and go into the sleeper, sit on the bunk and look around.
Does it look comfy and practical to you? Lift the bottom bunk and check out it's storage, and ask yourself if it's going to hold what you need it to.
Lift the hood, does everything look good to you? Look all around the truck, check out the fuel tank fill cap placements. They're placed completely different on a Volvo versus the Shakers (Freightliners).
After you do that, come back and ask more questions.
Hopefully all of these replies have helped you. Keep asking, we'll help where we can.
BigBob