aGPS tends to compliment
the regular GPS to help it acquire a lock more quickly. It's not inferior in any way, just an additional source of info your phone can use to get a lock. Once your phone has an initial lock it should be able to hold it without much issue at all.
So long story short
both the preceding two posts are probably correct.
It may be that in most cases you will want the extra aGPS data to help your phone get a lock (nearly all phones use aGPS + GPS). However, hiking in the mountains could mean you get some great unobstructed views of the satellites as well as almost no other background radio interference. Thus you may get a lock faster than in normal suburban or urban areas.