We're a little freaky about safety in my family, so we started with an infant seat. Everything I read said that larger babies are probably OK in a convertible seat, but that every newborn, small & large & in-between, is safest in an infant seat. So, we went with safer, even though it cost us extra $. I figured I'd never regret spending $ on the safest possible seat if we were in an accident...Thankfully, we weren't, and we passed the infant seat on to some friends whose baby should arrive in July.
Dd was 9.5 pounds & almost 22" at birth, and outgrew the seat (the max. height of 26", but not the max. weight) at 3 1/2 mos. Having the infant seat for that long allowed us to buy a convertible seat with slightly higher strap placement, so we should be able to keep her in this particular convertible seat to at least 1 year & 20 pounds, and hopefully longer. If we had started with a convertible seat, I think the straps on this one would have been too high, which would have meant buying a different convertible seat that may not have worked to 1 year & 20 lbs for our tall girl - If that was true, we still would have ended up buying 2 rear-facing seats, and would have missed out on the added safety of the infant seat for no good reason. I don't know if that sounds logical to anyone else, though!
The perfect solution is probably different for each baby...
The convenience factor didn't matter much to me before dd was born, since I was planning on slinging her ALL the time. We discovered, though, that is was much easier to buckle her in when the seat was cozy & warm inside a building, rather than getting her out of the sling & into the back seat of our small Subaru, and then properly into the seat, usually in the rain & gloom of a Seattle winter day. I still used the sling a lot, but I sure wasn't sorry to have the infant seat, too!
One final thought - It's very easy to click an infant seat into the base, or even to buckle the infant seat into a car without the base, but in my experience it can be MUCH more difficult to buckle a convertible seat into the car properly. It took 2 certified car seat technicians over 1 hour to get our convertible seat into our car, and there is NO WAY we're unbuckling it until we absolutely HAVE to. So, don't count on being able to move a convertible seat in & out of the car with any ease. It's not just a factor of knowing how to buckle it into your car; feeding the strap through & getting the correct tight fit & angle can be tricky no matter how many times you've done it.