That is the question. Some prefer 6s, some prefer 4s. There was once a gulf between building a 6s capable quad and a 4s capable quad. But that gap has now decreased to the cost and availability of a 6s lipo battery.
Basically, 6s draws less amps at high volts than 4s does at the lower volts. 4s motors need to work harder, so the motor KV will typically be around 2300-3000kv.
6s motors tend to be 1600-1800kv.
6s provides a more linear power delivery, meaning that you will be able to use the full power of the motors for longer on 6s, compared with 4s.
4s is still loved by many. Batteries are usually cheaper and more widely available. Check your local battery supplier and see which way you should go.
You can always build a 6s quad, and run it on 4s when you are starting out. Then buy 6s batteries and rip the sky apart!
Charge your lipo batteries to 4.2v per cell. Storage charge is 3.8v per cell. Run your batteries until they start to stay around 3.5v per cell when flying. HV whoop batteries are generally charged to 4.35v per cell, and can be flown down to 3.2v per cell. (e.g 2s lipo is 4.2v per cell x 2 cells = 8.4v fully charged. 7.0v time to land.)
Check out this video on how to charge those new lipo batteries.