Assisted GPS does not enhance accuracy but helps with availability. There is a tradeoff between reliability and availiability. Because users want to find the next car rental instead of landing a plane with zero vis, smartphones give you a position, even if it is very rough.
Differential GPS improves accuracy by comparing the receivers signal to one received by nearby receivers with fixed known position. If the signal carriers are used (for example 1575.42 MHz), an accuracy at centimeter level is possible. Carrier differential GPS either needs long observation time at fixed position (tripod and choke antenna) or permanent satellite reception while moving from reference station to the measured point (RTK). Carrier differential is available in handheld systems, but not in smartphones (to my knowledge).
Another variation of differential GPS is Augmentation Systems. These also compare their signal with nearby reference stations, but they do only for the modulation. This improves accuracy somewhat (8 meters DOP best case) by eliminating ionospheric effects common to both stations.
To cut a long story short, surveying with smartphones only works, if you average over a large number of measurements.