Tails was nothing but helpful. Even if he got lost, you were no worse off than you were if you were playing solo. As for Tails being better, in Sonic 2, Tails was worse than Sonic because he couldn't fly (if you were playing as him) and was slightly shorter, potentially missing you rings. In Sonic 3 Tails could fly, but then you had all those cool shield abilities that Sonic could use, like the fireball dash or the bubble bounce (not to mention Super Sonic). And Knuckles is badass.
Slippy, however, has no redeeming qualities whatsoever. By FAR the worst sidekick.
Slippy, however, has no redeeming qualities whatsoever. By FAR the worst sidekick.