Not really, A-GPS typically has a lower grade GPS chip, which makes a difference, and uses cell towers to look at cached data which is a bit older than what the GPS satellites can provide. It is predictive tracking. It is true that A-GPS can, but doesn't always, provide more accuracy within skyscraper canyons or similar geographic locations, but it can also show you on the top floor of a skyscraper rather than on the street below. In flatter terrain of suburbs and open country the accuracy of A-GPS is considered at least a bit less accurate than data received directly and solely from satellites. Outside the coverage area of cell towers, or when cell towers are few and far between, the inferior GPS chips often used in phones to provide A-GPS will show even more inaccuracy.

You can believe whatever marketing BS the phone makers and service providers feed you, but that doesn't make it true. In theory if A-GPS in a phone used a high quality chip set and an antenna with as high a gain as the antennas used in dedicated GPS only devices from Garmin or TomTom then A-GPS could be the better of the two. In reality, it highly depends on your usage, but GPS only devices have the over all edge.

As far as antennas go, I can tell you as a radio person that you can have the best rated, most expensive radio available, but if you attach an inferior antenna you will have terrible reception. On the other hand, even with a so so radio if you attach a well designed high gain antenna you will have as good of reception as can be expected for your location. It is all about the antenna. We always say if you have X amount to spend on a radio and antenna, then spend most of it on the antenna. So using an inferior GPS antenna within a phone is going to mean inferior reception of GPS signals which means it takes longer to get the data, if it can be received at all. If you are in Manhattan that probably isn't going to be a problem with A-GPS. If you are driving through rural America then you might need to have a backup mapping system.