Phase or Treads?
For most heroes, this isn't much of a question, but I'll take the time to talk about two cases where this actually comes up.
Sniper:
Sniper is unique in that he has very little actual defensive skills, which means Phase gives him a lot of movement based survivability, but he scales amazingly with attack speed such that he can demolish people trying to get to him just by attacking.
In this case, whether you get Treads or Phase depends on matchup, personal preference, and how much you can rely on your team to protect you. If they have a lot of gap closers, you'll want Phase, if they're mostly herpy derpy melee guys, you can get away with Treads. If your team has trouble peeling people off of you, get Phase. If your team is built for 4-protect-1, get Treads.
Templar Assassin:
In older days, TA had only one build and that was Bottle -> Phase -> Blink. She relied on her heavy damage Meld combos to demolish someone while keeping them in range forever with Traps + Phase. This build is extremely mobile, extremely aggressive combining both the Kiting and Chasing aspects of Phase into a cohesive playstyle.
Within the last year or two, an alternate style of TA has arisen that plays her like Luna or Morphling. She simply stacks stats (Treads, Boots, Yasha) and then goes to town. With a 1000 range range spammable mega slow, she has very little trouble keeping up with targets. She gets a lot of survivability from Refraction, AoE damage from Psi-Blade and Meld combos to finish off lonely targets. She is almost impossible to chase, unless you have lock down or Blink, because she's so fast. She can resist being initiated on very well with Refraction, and "dodge" key skills with Meld. I still prefer the Blink version, because it's more fun, but Stats-TA is very legit.