From what I can see the 340 lacks the car cradle and power lead. Additionally it only comes with Western Europe rather than full European coverage on the 350. I have a document supplied by Garmin, admittedly a couple of years ago, that lists the counties included in their regional mapping:
Full Europe: Albania, Andorra, Austria, Azores Islands, Belarus, Belgium, Bulgaria, Canary Islands, Channel Islands, Croatia, Czech Republic, Czech Republic, Denmark, Estonia, Finland, France, French Guiana, Germany, Gibraltar, Greece, Greece, Guadeloupe, Hungary, Iceland, Ireland, Isle of Man, Italy, Latvia, Liechtenstein, Lithuania, Luxembourg, Macedonia, Malta, Martinique, Madeira, Moldova, Netherlands, Northern Ireland, Norway, Poland, Portugal, Reunion, Romania, Saint Barthelemy, Serbia & Montenegro, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey, UK, Ukraine
Western Europe: Andorra, Azores, Belgium, Canary Islands, France, French Guiana, Guadeloupe, Ireland, Luxemburg, Madeira, Martinique, Monaco, Netherlands, Portugal, Reunion, Saint Barthelemy, Spain, UK, Switzerland, Italy, Austria, Germany, Liechtenstein, Greece, Denmark, Norway, Sweden, Finland
There maybe less memory in the 340, the specs don't say.