I'm just noticing what you said about her infant seat - is it the Onboard 35? That's a pretty tall infant seat, but if she has four inches left over her head at 15 months then she may not be as long in the torso as you think she is so you may be ok with a Britax seat if you still want to go that route.
Do you have a Buy Buy Baby in your area? They carry all the seats you mentioned AND will let you try them in your car. I would suggest going down there and spending some time seeing how your DD fits in them and how you like the features and narrow it down to a couple and then see which you like in the car best. But here's a quick rundown of the seats I mentioned:
The True Fit only RF to 35# and the adjuster button for the harness sits very low for RF. It has a continuous harness though which means you can pull the slack out from one shoulder or hip at a time and essentially bypass the adjuster. Some people love the continuous harness, some hate it. The Premiere version has an anti rebound bar. Here's a review:
http://carseatblog.com/3054/the-first-years-true-fit-premier-c670-review-very-nice/
The Radian has a low, narrow profile and is great if you have other passengers in the car and is very easy for loading and unloading a RF toddler because of the low sides. It can be difficult to install and does take up a lot of front to back space. There is an angle adjuster available now that is a foam wedge that goes underneath, specifically for the Radian that allows it to sit more upright, but does make it sit a little higher in the vehicle. Review:
http://carseatblog.com/14805/diono-radian-rxt/
The Complete Air has slightly twisty straps, a continuous harness, and a 40" stated height limit for rear facing as opposed to a 1" from the top of the shell rule. It's pretty easy to install, sometimes needs a noodle or a towel. Review:
http://carseatblog.com/3332/safety-1st-complete-air-with-air-protect-first-look-review/
The Pria is pretty similar to the Complete Air (Maxi Cosi and Safety 1st are both owned by Dorel) I haven't actually seen one in person yet, but here's a review:
http://carseatblog.com/11231/draft-dorel-maxi-cosi-pria-70-convertible-review/
The Primo Viaggio is brand new, it's expensive and tough to install with a seat belt (but a breeze with LATCH). So far folks seem to be pretty happy with it though, here's the review:
http://carseatblog.com/16793/peg-perego-primo-viaggio-sip-5-70-convertible-review/