Just buy a stand alone seat (meaning not a car seat/stroller combo). Neither the Chaperone stroller or the Cortina stroller thrill me.
Not that I advocate leaving baby in their car seat for a long time, but for quick in/out trips you simply can't beat having a frame stroller. Just snap the car seat on and go. Then follow it up with either a full featured lightweight alterrain (Valco Trimode is one example), or a full featured lightweight compact (Maclaren Quest or Britax Blink) depending on your lifestyle and you're done. For the in between times when all you have is a frame stroller but you don't want to leave baby in his/her car seat for too long, you ought to consider a good baby wearing device. Something like an Ergo or a basic wrap.
Now, if you absolutely insist on going with a boxed travel system, I would, I suppose, opt for the Cortina & KeyFit combo over the Britax combo.
p.s. unofficially, the Chicco KeyFit car seat works really well with the Graco adapter kit for the Valco trimode. If you blended the two together you would have the best of all worlds. As well, the Peg Perego Si works lovely with the Chicco KeyFit car seat. Again, blending the two together and you'd have a really nice set up.
HTH's'