So, I'm a complete newbie...but, have recently had experience with both. My son wanted goldfish...so we plunged right in and bought a 10 gallon. We did what a lot of people do...we set up the tank, let it sit for a week and thought it was good to go!
Of course, once we put in a few goldfish...they immediately got ich and within a couple of weeks, they were looking pretty sad. Long story, short...I ended up doing a traumatic fish-in cycle with fish dying right and left. It took forever for the tank to cycle because I wasn't able to get the fish to live long enough. I didn't even have a testing kit for anything other than ammonia at this point. Finally, one goldfish managed to survive my inexperience and the tank cycled. It is a super happy fish in its very own 10 gallon tank.
We learned a lot..first and foremost that our personalities do best with cycling WITHOUT the fish in the tank. I really think it is a personality thing. I'm thinking that some people simply enjoy the challenge.
When my son decided that he wanted tropical fish, we bought a used 30 gallon tank and got serious about a fishless cyle. I bought the API test kit and used filter media and decorations from the goldfish tank to seed it. I don't know how people are cycling tanks in a couple of weeks with seeded material, though? We are at about a month in and I think we are just now getting to the end of the cycle. But, for us, it has been so much less stressful and I can't wait to be able to fully stock it all at once when we finally reach the finish line!
So, really, if you know what you are doing and enjoy the challenge...fish-in is probably fine and could even be faster. But, if you are a newbie or just want a more low-key experience, I think it is the fishless cyle for sure!