The biggest one to me is that a booster seat is not regulated, and many of the 3-in-1 car seats (baby-toddler-booster) tested as unsatisfactory or would only work with SOME cars, and not with all. If I'm going to spend money on a car seat, I want it to work in no matter car seat I use. Also since they are talking about forcing regulations in the next year or so, the car seat you get NOW may be no good in a couple of years. I'll include a link to the place that tested the boosters and their results so if you do go booster you can pick a good one.
Alright, I've used 3 convertibles so far. The first is a Britax Decathlon, it goes rear-facing until 33lbs and front-facing until 65lbs, has side-impact testing, and sits high. Some people don't like the room it takes up, but I like the fact that it is high enough she can see out the back seat. It's also pretty plush and comfortable. We took it on an airplane and it fit, but you had to pull up the arm-rests.
The second was a Safety First 3-in-1 I believe, we were still using it in the rear-facing position. Before the accident the latch for the seatbelt was stiff after just a week or so of use and was getting harder to manage. After the accident, because of the severity of the accident, I was able to get them to replace it.
I replaced it with a Graco-My-Ride. The cost was about $169 at the time, it's $129 at Wal-mart now and you can get it under $150 most places. It faces rear for 40lbs (for my daughter, that means she'll be rear-facing probably until 4 years old in that one) and faces forwards to 65lbs. It's not as comfortable as the Britax, but has good side-impact and my daughter likes the cup-holders. :P Also the leveler is easier to use than on the Britax, it's easier to rethread when they start to outgrow it (some of the better Britax car seats you don't have to rethread at all to fix the shoulder straps as they grow, which I think is wonderful) and sits high (again, she likes to look out the back window.) I like it as much as I like the Britax, maybe even a little more.
Now, if I had the means and needed a convertible I'd look at the Radian Sunshine. It faces backwards to 45lbs, forwards to 80lbs, isn't as wide, has good side-impact, a steel frame, and a longer-expiration. The Britax has a 5-6 year expiration on the plastic (you should replace after that time) and the Graco has a 6 year expiration. I think the Radian has an 8 year expiration as well as allowing you to use the LATCH system longer, which is safer. It runs close to $300 though. The negative for that one of course is that people have complained about trying to get it to sit in the rear position laying at the 45degree angle like it's supposed to while others say it didn't give them any trouble in smaller cars even, so I'd want to try it out before buying. Still, that's the one I'd lean towards since you'd basically be using it for the rest of your child's car-seat needs.