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Should be less than 10 lbs, 6-8 empty is best.
4 Wheels vs. 2 Wheels
The hot thing now seems to be hard sided spinners (4 wheels)
I've seen too many people with spinners crashing into seats as they try to control the bag going down the aisle on a plane.
4 wheels take up a lot of space that you could otherwise use for your contents. I prefer 2 wheels. They are usually recessed so you get more space and are easier to control.
Hard Luggage vs. Soft Luggage:
This is more a matter of personal preference. Soft carry-on luggage allows you to really maximize storage and squeeze in that extra inch of room. Typically there’s an outer pocket where I store papers and odds and ends. Soft suitcases tend to be heavier but hold up better to wear and tear.
Hard carry on luggage is sleeker and has a more distinctive, luxurious look. Thin polycarbonate hardcases can often be lighter, but in my experience, scratch easily.
Whichever you choose, make sure there are plenty of internal pockets for organization.
If you always seem to be forced to gate check, you may want to opt for a soft-sided bag. They generally hold up best to wear and tear, though hard-shell bags are adopting more aesthetically durable materials as their popularity grows. Hardside luggage is generally a little more protective of what's inside, while softside bags may allow you to squeeze in an extra bit of baggage for the way home because the fabric has that added give.
Carry-on Backpack vs. Carry-on Suitcase
If you tend to over-pack, a carry-on suitcase, which is larger and has more room will be best.
TSA Lock - Allows TSA to open the bag when it is locked.
You only need to lock your bag if your are carrying firearms.
Carry-on luggage Company Ratings
Pack Like a Pro: Ultimate Guide to the Best Packing Cubes 
Carry On Luggage - Amazon
Choosing the Best Carry-On Luggage (Updated for 2019!) | Along for the Trip