Seeking Recommended Tall-back Booster Seat Recommendations

I just switched my daughter from the britax marathon to the britax parkway booster she's 5 & loves it.... she is just a little over 40 lbs and tall. one of the things that I really loved about the britax is the secure guard it looks like a belt in the middle between the legs that works w/the seatbelt (works with the vehicle safety belt to prevent the child from sliding under the lap-belt portion of the safety belt (submarining) during impact, thus minimizing the risk of abdominal )injury. I purchased the booster from chitchatbaby.com w/a coupon it came to 95.99 free s/h no tax.

http://www.britaxusa.com/car-seats/parkway-sg

Check out
www.seatcheck.org for specific laws for your state.

Do not be sucked into the myth that expensive is safer as far as car seats go--they ALL meet the safety requirements.

Below are several that are likely popular because of safety (though Britax are more costly).

We personally have this one and LOVE it. Our son is a similar size to yours (3.5, 39 inches, 39 lbs) and does great in it. We're expecting to have it for the next few years since it's convertible and love that it has the 5 pt harness right now.
http://www.target.com/Graco-###-###-####-Nautilus-Seat-Wi...=

These air ones have been well-received by experts because of their design:
http://www.target.com/Complete-Air-Convertible-Car-Seat/d...=

I don't think you can ever go wrong with anything Britax personally.
http://www.target.com/Britax-Frontier-Booster-Car-Seat/dp...=

At only 3.5yrs old, I would keep him in a harnessed car seat. While he may be big enough for a booster seat, he's not mature enough mentally or physically for that responsibility.
A Graco Nautilus is $150, will harness him for at least 3 more years, and then becomes a high back booster to 100lbs, and eventually a no back booster. The no back booster is good for 9yrs, so he will never need another seat, and it can be used for any children younger than he is once he outgrows it. It looks very similar to the Turbobooster, but with a harness. Its a great seat that will not only keep him safer in the harness, but be the last seat he needs, and its reasonably priced.

I know you need it as a backup seat in a second vehicle, if he's only rarely in there, then maybe a Graco Turbobooster would work because they fit smaller children well, but especially make sure its always high back. But if its more than occasionally, a five point harness is the safest.

I don't know a good one for occasional use but we got our son the Britax Frontier. He can still be in the 5 point harness for a while and it converts to a booster. It's a great carseat but it is around $270 I think. More than what you probably want to spend on an occasional seat.