DD (12.5 months) will soon outgrow her Safeseat, and I'm starting to research convertible seats for our next step. I'm hoping you all can help me out with real-life feedback!

We live in NYC and don't own a car; we use a stroller and public transportation 99% of the time. So the seat will mainly be used for occasional cab rides (when the weather is too bad to walk to the pediatrician), in rental cars to Grandma's house a few times a year, and in planes a few times a year. The main things I'm looking for:

-easy, FAST installation using seat belt (since there's no guarantee any car we use it in will have LATCH, and I hate fumbling with the belt clip on the Safeseat while the cabdriver gets impatient)
-not too cumbersome/heavy, in case we have to haul it through an airport, and again for ease of installation and removal
-Weight limits aren't so important; DD is small (19.5 lbs, 29 inches at 12 months) and will outgrow the Safeseat in height long before she reaches the weight limit.

I know people LOVE their Britaxes, but I'm hesitant to spend $$$$$ on a seat we'll use 12-15 times a year at most. (And as I said above, we clearly don't need a 60-lb weight limit!) But I also don't want to get a cheaper seat that I have to spend 10 minutes fussing with to install properly. Thoughts? (And thanks in advance!)