I would go over to a large Macy's and check some of them out. It really helps to feel them out - see how heavy they are, how well the wheels move, check out the interior compartments on each bag. Also, keep in mind that Macy's constantly runs sales. We signed up for pieces of Samsonite luggage that have "regular prices" betwen $200-800 but they are almost constantly on sale for between $150-400. I liked the wheels, sizes, weights, and handle placement on these particular bags better than any of the other ones i saw.
Also, remember that right now you might only need a couple of roll-aboard carry-on sized bags, but in the future when you have a bunch of kids to pack for in addition to yourselves you may want larger pieces - may as well get them now if you want everything to match, luggage gets discontinued frequently.
My best advise is to not get black luggage - that is what most people have. If you have an unusual color, it's a lot easier to spot it on the baggage claim.
Definitely get ones that have the "360-degree" wheels... the others are a pain to use.
Go to the store, lift them up and check the weights. My parents bought a really big one (larger than you typically find in luggage sets), but then realized that the suitcase weighed so much by itself that they couldn't fill it without going over standard weight limits. Also, some models are heavier by themselves than others.
Hard cased luggage used to be heavy, but the latest materials allow them to be really light... often lighter than soft-sided cases. This also means you could get a larger case without worrying about the case itself using all your allowed weight. They can protect your items better, but they also aren't as flexible for really squishing items into them as soft-sided cases. I've always had soft-sided, but have lately started considering switching to the hard-cases.
Make sure you get a really nice, light 21" carry-on suitcase. For many trips you can get by with just the carry on case, which is more important since airlines started charging for checked bags. I've gone away for over a week with just carry-on by packing efficiently. So this will likely be your most used piece.
And like delujm0 said, don't get black or dark/navy blue. They are really common and it makes it much more difficult to find your luggage. The more unusual the pattern or color, the easier it will be to spot amid everyone else's luggage. I have a bright blue set that is fairly easy to spot. I also added rainbow colored ribbons tied around the handles and have monkey & duck shaped luggage tags to make them further stand out.
To piggyback off of @nicoann your carry on will probably be the most used piece- so choose wisely. Also make SURE that the one you pick actually fits the carry on size allowance - several of them are slightly too large, or are fine until you extend them, at which point they are above the size allowance and have to be checked.
I advise getting one with very few inner pockets. You'll fit more stuff if you just have a big empty box. I have traveled to Europe on 3 separate occassions for 2 weeks at a time using only a carry-on bag. It can be done if you have the right one. FI and i actually registered for two matching 21'' carry on bags - because we only have one old one right now and when we travel together we fight over it. You might really each need your own if you want to avoid checking bags and travel regularly. Or get one standard 21'' carry on and one 21'' wheeled duffel bag or something like that.
We just upgraded our luggage and went with Delsey. I love them. I really loved the Samsonite too but it was just alittle small for me. Like others have said 360degree wheels are key! I do alot of traveling for work so we splurged and just bought my suitcase with brithday money and Macy's sales. I can't believe I ever traveled with my old suitcase.
Really testing them out is key. I have my luggage as standard fabric bg and FI picked his bags as hardshell beause that's what he likes. It's ok to mix it up if you both like different brands. I would suggest going against black colors, a colored bag is much easier to spot at the airport. In the same topic though, don't get to crazy with colors bright colors will show dirt very easily.
We did register for 29" suitcases, but I have done some international travel and the larger suitcases are nice for longer trips. I would also recommend thinking of pieces beyond suitcases. We added a rolling garment bag to our registry, thinking it might be nice for traveling to wedding, funerals, parties, etc...