There is no difference in quality but the performance is different depending on your usage scenarios. HD 3450 will handle one 1080p decoding excellently, but you can't do much else that needs GPU power at the same time. For instance, if you have Windows DreamScene running in the background, the HD playback can become choppy. Or if you have a dual monitor, you may also experience the difficulty doing anything that requires GPU work while playing 1080p movie in the other monitor.
So if you do one thing at a time, 3450 will suffice. But if you want to do something else while watching HD movies or have two monitors to hook up, then 3650 will be much better.