There are a number of newer convertibles on the market you might consider.
The Graco MyRide 65 offers a high rear facing weight of 40lbs and a FF weight limit of 65.
The Sunshine Kids Radians offer between a 40-45lb RF limit and a 60-80 FF limit depending on the model. These also fold for travel and have rear facing tethers. These are also the narrowest convertibles on the market and are great for trying to fit multiple seats side by side.
The CompleteAir is fairly new with a 40lb RF limit, and a 50lb FF limit. These are known for high shoulder strap heights, great if you have long torsos on your kids.
Overall the safest seat for your child is the one that your child fits best in, that fits your car the best and that you can install and use correctly every time. I would suggest that you visit a retailer like BabiesRUs and try out some of their seats. They should even allow you to take one out to your car for fitting purposes.
Additionally, I would urge you to become familiar with the latest suggestions and guidelines about Extended Rear Facing your children. We know now that rear facing is safer for children till about 4 years old, or at least to the limits of your child seat. There is a ton of great information about rear facing and it's benefits on the internet. I would recommend safekids.org or carseat.org or car-seat.org for more information!
Hope that helps!