Between those two, I lean towards the gray tree frogs. They are a little less jumpy and if you can collect them yourself, they will usually be in better health. Even better than either of those as a first tree frog would be a captive bred White's tree frog, although they grow larger than the other two and need more room.
Green tree frog care sheets:
http://www.froggie.info/green/index.htm
http://centralpets.com/pages/critterpages/reptiles/frogs/FRG2502.shtml
http://www.anapsid.org/greentreefrog.html
http://www.amphibiancare.com/frogs/caresheets/greentreefrog.html
Gray tree frog care sheets:
http://centralpets.com/pages/critterpages/reptiles/frogs/FRG2503.shtml
http://www.flumpa.com/frogs.htm
http://www.amphibiancare.com/frogs/caresheets/graytreefrog.html
White's tree frog care sheets:
http://www.froggie.info/whites/index.htm
http://allaboutfrogs.org/info/species/whites.html
http://www.hylid.clara.co.uk/caer.htm
http://www.amphibiancare.com/frogs/caresheets/whitestreefrog.html
Good luck,
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Devin Edmonds
devin@amphibiancare.com
www.amphibiancare.com