GPS Module specs really don't vary that much.
The place they vary is really the number of correlators onboard the module memory, time to first fix (TTFF), power consumption, receiver sensitivity and IO options. Other things to watch for: Can it utilise other signals available (such as GSM/3G) to help rough position itself in very low signal strength areas?
Does it support an active antenna? Note that you have to be careful about using an active antenna in close proximity to the unit.
The datum used may vary which could affect local accuracy, although they all seem to use WGS-84 nowadays.
The other thing that varies is price and footprint :)
All GPS modules should require 3 satellites for a fix, 4 for a 'good' fix as it's inherent in the method used. Accuracy is really much the same across the modules I have investigated (I use GPS modules in some products). There are two accuracy definitions: Absolute accuracy and differential accuracy. The usual specs are