Which do you prefer?
I've got both.
The duffle is trendier, red, and easier to steer, though it can tip over. But it needs 4 locks(!) and things get lost inside.(I literally lost a pair of black flip-flops behind an inside pocket on my last trip and found them only after I got home.)
The soft-sided suitcase is an old black workhorse that needs only 2 locks and lets you see everything. It weighs a smidge more then the duffle and holds a bit less.
Trying to decide for next week.

I can't figure how a duffle is easy to steer - mine is not. I think I have tried all types of luggage starting with a backpack as a teen and now have come full circle back !
But if your black suitcase is a wheelie then I prefer that to a duffle for plane travel. I like my duffle when I'm off in the car to the cottage or something and I just sort of throw everything in it.

The backpack is great for hands free travel but it's now hard to find mid size simple ones. Most of them look like something you need to climb Mount Everest with!
Bon Voyage

To me, duffles are too softsided. I'm always afraid they're going to rip and tear.

I also find that when packing a wheeled duffle, you put everything in so that the heavier items are at the wheel base so it doesn't topple over, but when you have to pick it up or lay it down, everything shifts.

I've done a wheeled backpack and that works OK, but like Christo says, they tend to have so many little compartments that are useless that you can't get 1/2 of what you need in it.

So, I usually stick with a wheeled upright. It's sturdier than the duffle, things don't shift if you use the cross straps inside, and it fits a maximum of gear.

Of course, with all my camera gear, I now take a wheeled upright (checked), a wheeled camera bag (carry on), and a non-wheeled duffle bag for "overage" on the return trip (it's the only bag that fits easily in the upright for the flight out).

Both the duffle and the suitcase have wheels then?

Sounds like you've traveled with each of them before? Which did you like better?

Yes, both have wheels.
Had to buy the duffle for a tour and it does feel very sporty. And I love that it's very lightweight and easy to spot at the airport. But it's harder to live out of.
The suitcase is an oldie but goodie-If I take it maybe I'll paint a big orange "G" on it (and on DH's identical one) so we can find it on the carousel.
Hmmm- sounds like old-faithful wins.

Luggage is great ! Maybe because it reminds me of traveling - or maybe I just like luggage !

The black suitcases are ubiquitous on the carousels so I learned a simple identification method years ago and that is to simply attach a bit of coloured cord, wool or something to a handle. I guess painting would work but seems a little drastic - no ?

I also carry a compact duffle inside my suitcase for the overage I buy while away. Sometimes I just buy a cheap bag on the road before returning if I need it. Maybe that's why I have so many bags at home.

Absolutely, no question, I would go with the tried & true suitcase.

Also myself I don't like duffle bags on wheels... too floppy somehow.

Where's the trip to? That might also make a difference in my reply. For a beach vacation when you're trying to cram in beach towels, etc. I might go with a duffle. Europe? Definitely a normal suitcase.

I'm not sure if you were kidding or not, but I wouldn't paint big orange letters on my suitcase Kind of embarrassing, don't ya think?

Just tie a colored something on the handle &/or use a distinctive luggage tag.

We have bright colored luggage straps, which do the job well.

OK-
Maybe not an orange "G". Maybe an orange stripe.
I'm past being embarrassed by things like that!
We used to use those big, heavy, tan, wheel-less, hardsided suitcases (remember them?) and put lime green stickum circles on them.
Actually luggage straps would probably work best!
We're off to Greece - part land, part small-ship.

I've become a fan of packing cubes, and they work well in duffles. The first ones I got were from Magellan's. I've found some from Rick Steves that, at 7 oz. for three, are only half the weight: http://tinyurl.com/f83cy

Those cubes would work great in the trunk of our Miata, which is not suitcase friendly.