First of all tell us the planned use of the luggage. Are you planning to carry suits? Is this for business or holiday? Will you go there once and that's it with the big luggage? Or do you often go on 2 week trips? Where are you going and how? Lux travel or on a shoe-string budget? Where are you going and how?
What is your budget (the most limiting factor)? $100? $500? $1000? $1000+? You can buy something that lasts you many trips (Tumi, Rimowa, Swissarmy, etc) or something that will last 2 weeks for much cheaper (ebags). You going to stay in a hostel or a 5 star? A rucksack fits in better in a hostel than a 5 star. Forget leather, as all luggage will be mishandled somewhere along the way. Leather is only for carry-on.
Hardside or softside? Depends on what you will carry. Anything fragile, would recommend a hardside and bring bubble wrap with you. Otherwise I would go softside since they usually weigh less. Tumi and Rimowa make some lighter hardside luggage - actually it's a softside/hardside hybrid. If you will be carrying suits or dresses, would recommend a suiter or a folding suiter sleeve.
Are you planning to travel by air exclusively? Don't forget the airline size & weight restrictions (different with each and much smaller & lighter once inside Europe). You don't want to pay more in fees than you paid for your luggage.
Train travel? Not easy to lug a 70+ lb big piece of luggage up and down stairs in smaller train stations lacking elevators or running to catch a connecting train on the other side of the station. If you are traveling a lot by train, this will get tiresome quickly. Europe is not the US. There's a lot of older infrastructure. Plus if it is too big, you won't have a place to put it inside the train. 2 pieces of luggage? Keep in mind that if they are both heavy, you will have to lift one at a time up the stairs. The concern here is that some train stations you have petty thieves. And they WILL run off with your unattended luggage. I have seen it quite a few times. Also there are frequent thefts of unattended luggage on the train themselves. I often hear over the loudspeaker of theft warnings. Advise you sit somewhere you can keep an eye on them (esp. if they look more expensive).
Rental car? Rarely will you find a cheap car rental in Europe that you find in the US and as a consequence, you will probably rent a smaller car. And I do mean smaller. Not so many SUVs or big cars in Europe as you find in the US. If you are expecting a full size, expect to pay 3-4x compared to the US, at a minimum. Don't forget insurance. Your credit card/ car insurance policy usually only covers continental US. Gas is at least $4/gal, so keep that in mind too. Driving in the UK? They drive on the opposite side of the road. Italy or Spain? Forget about it unless you've driven there (or Mexico City, similar) before. Parking can be far more of a hassle than you find in NYC. The infrastructure in older cities (or old city areas) were made well before cars were invented and some places remain largely unchanged or little has been done to adapt them to cars.
Answer these questions and your life in those 2 weeks will become much easier. Fail to consider them... well, I will let you imagine.