Some things to keep in mind when choosing a convertible seat:
1) What is the height/weight limit for rear-facing? The AAP recommends all children rear-face until at least the age of 2, so take those restrictions into mind when choosing a seat. If you feel your baby will reach 35lbs (which is the basic limit for most seats) before 2 years of age, consider going with a higher limit seat even if it costs a bit more.
2) How easy will it be to clean? Does the seat cover come off? Does the manufacturer sell replacements?
3) How likely are the straps to twist? This is my number one annoyance with car seats! The straps aren't as effective or safe when they're twisted all around into thin slivers of material but some are just impossible to keep from getting that way. Reading consumer reviews (try Amazon.com for some of those, or even here) can help with getting an idea of what you're looking for here.
4) Last but not least, think about how likely is it to fit in your car. I had a Saturn Ion before getting my Ford Fusion and the seat my daughter uses now would not rear-face in the Ion. There's just no way it would have fit. I didn't switch cars because of that, since I did so before she moved to a convertible, but it's definitely something to keep in mind. You should be able to buy a seat and install it and have no issues returning it. Keep that in mind when you choose a store to purchase one from.
All that said, we use the Safety 1st/Cosco Alpha Omega Elite. It rear-faces to 35lbs and turns into a 5-point harness at 40lbs (which is also an important feature). It comes with a sippy cup holder and moveable armrests. Costco has it on their website for $89, I believe, but we got it for even cheaper in the store. The straps do occasionally get twisted but I make sure to fix them before taking my daughter out so it doesn't get out of control. She is 17 months and still rear-facing and has no issues with it whatsoever.
Also something to remember: "ratings" for car seats are useless if they're not properly installed in the car or if they don't fit the car correctly. When you buy the new seat, make an appointment with your local fire department or health department's safety division to have it properly installed. Take notes and pointers from the person who installs it! It was amazing to see the things that you just don't know while installing a seat; most people think you just buckle it in and you're off. Not so! Check out this link to get an idea of where you can go: http://www.michigansafekids.org/passenger-fitting.php
Good luck!