I grew up near Philadelphia, and while I'm not currently active in Real Estate Investing, I would say that Northeast Philly is fine in terms of crime. I even think a lot of cops live in the neighborhoods you mentioned, though I'm not 100% sure.
I'd still be generally careful and talk to locals in any of the neighborhoods you mentioned before buying. Philly is weird in that a few blocks can be all that separate you from an amazing part of the city absolute hell. Case and point, near 2nd and Girard is becoming a popular and gentrifying part of the city(I've even heard rumors of skyscrapers being built near there), but I know of someone who got mugged with a sawed off shotgun at 6th and Girard. No matter what you do, look very closely at not just the neighborhood, but part of the neighborhood you're in. Especially true for somewhere like Kensington.
It's hard to decide the tradeoff you mentioned, however, I can easily envision parts of Kensington and Rittenhouse appreciating rapidly very quickly.