Hi and welcome to car-seat.org!
Are you set on an infant seat? How long does the MyRide have before expiration? I am fairly confident the Size4Me does, too, but I really like the fit of a newborn in a MyRide, and because it is narrow at the base and flares up higher, it tends to work fairly well for booster or combo seats next to it.
How is your 5 year old maturity-wise? What are you goals for harnessing? I know with my own 5.5 year old, we do occasional booster rides, and I'm thinking towards full-time booster use, although I'm not there yet. If you think he'd do well in a booster, I'd look at some of the more narrow ones, like the Graco Turbobooster, and see how that works with either the KeyFit or the MyRide. IF he's not booster ready, I'd look at the Defender or Maestro, or maybe consider putting one or 2 of them in the 3rd row (you'd have to do side-by-side in the 2nd row if you wanted access to the 3rd, so putting both back there may be a better plan.