I have had my HTC One V, a low-end smartphone, from Virgin Mobile for a while now, and I have noticed that it is running slower and slower, probably from all the updates and bloatware. I have not been successful in rooting it and installing a custom ROM.
I have been doing some research and I just ordered a 32GB Nexus 5 inbound from Google Play. I combined that with a free T Mobile prepaid micro sim. When the phone and card arrive, I can activate it online with a T Mobile prepaid plan at $ 30/month: 100 minutes talk time, unlimited email, text and Web. I have been keeping track over the past year, and I have not had a single month in which I used more than 100 minutes talk time.
I have been so pleased with my Nexus 7 tablet, and especially the Google Now functionality, that this pushed me towards the Nexus 5 phone. Plus, it will run pure Android with no manufacturer apps or bloatware.
We will see how it works, although the online reviews are positive.