GV Mobile+ and GV Connect are two different applications that allow you to use Google Voice applications on your iPhone. The first difference between them to be mentioned is that the GV connect shows just 11 messages that are received most recently, while the GV Mobile displays 15 most recent of the messages received.
The GV connect would allow you as a user to be able to browse through the previously received messages, but the GV Mobile does not give a chance to this. Hence, you can be able to also easily browse through the voice mail, as the GV connect seems more flexible to use.
Another difference between the two is that the GV Mobile presents a bigger font with more space in use, while the GV connect presents smaller fonts in such a way that you have more texts to view on the screen. The GV Mobile has more sales, most likely due to its nice graphics.