I have a very good experience with Garmin eTrex Legend HCx - here and here they write that HCx has a very sensitive chip, that works in forest, deep valleys and in cities - and I can only confirm - with HCx I was able to get my position in plane!
How sensitive is eTrex 30, related to HCx? Have you been able to catch satellites in room / plane / in a narrow street? eTrex 30 doesn't say to be that sensitive at garmin website.
(Could HCx then be more precise/usable in these conditions despite eTrex 30 supporting GLONASS?)