Both are great. Lewa is definitely wilder feeling. Ol Pejeta is fenced, and you will probably see the fence at some, or several points, as well as the lights of town, and there are more other vehicles around. So definitely less wild and remote.
However what is incredibly cool at Ol Pejeta, and especially if your family includes kids, is the educational aspect. Each camp runs their own game drives out in the conservancy, but there are several activities run by the conservancy itself (usually at extra cost) which are really great. You can visit/feed Baraka the blind rhino (great for younger kids), see the last two Northern White Rhinos ON THE PLANET, visit the small education centre, even go running with the anti poaching dogs (for fit adults only). So I think Ol Pejeta is really great for learning about the more practical side of running a conservancy and learning about how everything really works, rather than just looking at the wildlife. Even if you just go look at one of the elephant corridor openings and you can see how they are constructed to allow certain animals in and out but to keep the rhinos in.
In general I would say most of the accommodation on Ol Pejeta is slightly more low key than Lewa, but still nice. Sweetwaters is on the busier side, but the tented camps are in the more remote part where there are fewer visitors. The Porini camp in particular, is in the most remote part of Ol Pejeta so you're a lot less likely to see other visitors near camp there.
Alexandra- Extraordinary Africa