I love our Osprey Poco Premium I bought for our first child. I just broke it out of storage as our second is now old enough for it. It has 400+mi on it and it is best in class IMO.
Here's a rather memorable hike we did with it:http://adventurewithalex.com/2013/07/25/alexs-first-4000-footer-mount-washington
From the cheapest model in the line to the most expensive I think they are all great. In deciding how much to spend I would suggest thinking about;
1) How many miles do you want to put on it?
2) Will you have a 2nd kid at some point?
Even with out a 2nd kid re-sell value of these on Craigslist/Ebay is quite high if you take care of it... so IMO I'm happy we bought the best one we could at the time.http://www.ems.com/brands/osprey/all-osprey/kids-carriers/
*Disclaimer I work for EMS so I'll tell you Club Day is coming up and you could get this at 20% off plus 10% back through EMS Rewards... flippin' good deal IMO