I liked both shows equally. I understand that many people like to compare two shows and states that How I Met Your Mother copied friends. Characters are similar: Ross = Ted, Rachel = Robin, Monica = Lily, Chandler = Marshall, Joey = Barney. They also had several shared storylines as well.
1. Both of them play music: Phoebe plays guitar and Tracy plays ukulele and bass guitar.
2. Both of them were into causes: Phoebe was into Animal Rights, and Tracy was into Ending Poverty.
3. Both of them are the nicest girl among three and like to help people. Rachel/Robin tend to be a brat sometimes and Monica/Lily tend to be over controlling sometimes. Tracy/Phoebe is the most selfless and kindest girls among the three.
4. Both of them are quirky: Phoebe runs so funny and Tracy sings English muffin song.