Strictly for sport flying I would go with a Tri. I have a Tri (not with Arducopter) and it is pretty sporty. But, my quad is is much more agile and fun for sport flying. I imagine a Y6 would be fine too but the extra weight would probably work against you.
I 'm sorry but the Y6 is powerfull than only 3 motors and you will have more fun with it. (no mecanical part in a Y6) so you can check it !!!!!!!!
ya y6 will be the better way to go & you do not need negitiv props if you poket can aford it
For super smoothness I like a Large Tri.. 30 sec video..
Also.. I slide this thing around on the ground all the time.. Y6 would break more..
Yes you do need reverse pitch props with a Y6. You don't need them with a regular tricopter.
I guess it all depends on what you consider "sport flying". For agility and acrobatics, I am pretty sure a quad will be the best. For power and redundancy, a Y6 is best. For a less expensive, smoother option, I would go with a large tricopter.
Here is my tricopter. It only uses cheap gyros (4) and no control board etc. One FPV video from it as well. The other video shows how acrobatic a quad can be. I have one built just like the one in the video but my skills aren't that good.
the frame im looking at is on ebay, search for y6 on ebay , ive decided on building a tri as i have a frame i built a while ago and didnt finish, loaded the software to the board, couldnt get the tail servo to work