I went with the Snugride because when I purchased (1.5 yrs ago) the Snugride had a higher safety rating. This was mainly due to the lower weight limit, not so much different construction. I also liked that it was slightly more compact than the Safeseat. As my daughter grew I found I was more than ready to give up the infant carrier by the time my daughter reached the weight limit. A carrier is great in the first few months when you can haul them around everywhere, they'll continue sleeping in the carrier, and they're not too heavy. But once my daughter got into the 6-9 month range, she didn't continue to sleep in the carrier once I took it out of the car, and she was getting much too heavy to tote around in that thing. I was ready to switch to her infant/baby car seat (we have the Brittax Roundabout) at that point, and never looked back.
Also, someone made a comment about a belt lock not coming with the car seat, but mine came with every part needed for proper installation. Plus, most modern vehicles are outfitted with the LATCH system, which is safer and more secure than a seatbelt installation, making the point moot.
If I had to do it over again, I would have made the same decision, focusing on the most compact carrier (bigger ones are tougher to lug around), with the highest safety rating, regardless of weight limit because I was ready to switch her by 18 lbs or so anyway--you can't physically carry them in the carrier much beyond that point!