Looking at the school comparison, a few things to note:
The schools are roughly the same size, although Mann is slightly larger.
Mann has higher test scores and is classified as a Reward school; Hearst has lower test scores and is classified as a Rising school.
Hearst has a higher percentage of ELL, SPED, economically disadvantaged, and non-white students than Mann.
Mann has a higher IB percentage.
Mann feeds to Hardy MS, while Hearst feeds to Deal MS.
http://profiles.dcps.dc.gov/Compare.aspx?tab=1&school=258,273
I know people who are happy with each school, but none who have sent kids to both. Mann has historically been an excellent school, but Hearst seems to be on the upswing. Both have recently renovated facilities--Hearst just got a new gym. I think you really need to visit the schools and get a sense of the schools and communities. I would be comfortable sending my kid to either school.