In Regards to your inquiry, Both phones I have tried out and settled on the htc 626s(tmobile variant). The differences in software are huge. The grand prime lags due to the UI, while the HTC 626s lags from overusing or not closing active apps(relatively easy to do). Also, due to this I recommend the HTC considering while it may have a Snapdragon 210, its at least lagging while overexerting its hardware. The Grand Prime lagged while just from general use even though it has the 410 processor. The HTC makes most of its hardware and its software runs smoothly hence why I settled on the HTC over the grand prime. In addition, or more as a side note is the screen resolution on the HTC is 720p whereas, the Grand prime has i believe 940x480(or something along those lines).
Hope this helped in your decision process.
12-23-2015 04:17 AM