agree with rick-ray, personal preference. myself, i don't do near as much emailing/texting, so i am happy with the pearl. two people in my office have the curve and they like it for the full qwerty. they like the simply everything plan. we crunched numbers the other day and i pay about $5 less per month than they do - which is negligible.
can't speak to sprint billing practices. i have combine billing with ATT and don't have any problems. good luck.