Here is my 2 cents. I am an average player and only play pickup games, so advanced guys will have better advice.
I have tried BOTF and BOTS builds with ooze, spit and hybrid.
With BOTS i was unstoppable and pretty much marched around like I owned the place, got to the Citadel in record time, had tons of early kills with infinite spits and foul graps. However that was only against noobs. Against good players, i was crushed, i had no staying power and my life would just evaporate. If I had a better team, that worked together, that might make it work and be worthwhile or if was better myself.
Here is what the problem is. Against good players, you never get a 1 on 1 and if you do, they run away. And good players are usually fast as hell too. They can get away from you and you can't get away from them. Good players also port and gank.
With BOTF i was also slow and mana starved in addition, sure i could stay alive a little longer but was even worse off. I felt like an immobile tower just waiting to get killed.
Unless you are on a team with good players and who are aware, you must take the Swift Anklet, otherwise you are rolling the dice. You have to be able to get away when a 2nd DG shows up and chase down one that is fleeing you or your teamate. You can't count on ports cause it will get interupted by good players.
Anyway, i noticed that i win a lot more with Swift. If i had a premade team, i would use BOTS. I pickup the swift anklet, and even then sometimes other DGs will be faster, so i might get the boots too.