I bought an Oregon 400t for my son, and I have the Montana 650t. Im relatively new to both units, but here is what I know so far. The brightness on the Oregon 400t is definitely a problem. Not bright enough....PERIOD. The 650 does a much better job here. My concern was that the Montana 650t would be way to big. Now that I have it in hand, it is large, but I wanted the larger screen and was willing to make the sacrifice in bulk. I also feel that the touch screen on the Montana 650 is better than the Oregon 400 from what I can tell. The 400t doesn't have some of the nicer features that the Montana has. Ive heard the newer Oregon units had brighter screens, so they may be ok but I have no experience with them. I haven't had any lock ups with either unit yet. From some research I did, some were having lock up problems because they had 32G memory cards in their units. When they dropped to a 16g card, they had better luck. Again its just something I read, I have no first hand experience there.
After two weeks use (not very much obviously) im liking the Montana 650t much better than the Oregon 400. BUT with that said, if don't want the bulk of the Montana, I think the newer Oregon has a lot of great features, to include an 8 mp camera if you get the right model. The Montana only had a 5MP camera. Not much help, but maybe a little anyways...
One last thing....whichever GPS you get, make sure your running the most up to date software update. I updated both of my units before I even messed with them (and I bought both units used). I got the 400T for 103.00 on ebay, and then got the Montana for another 376.00, also from ebay. That can be a risk because your never sure what your getting, could be refurbished, etc...but If you have priced the units, you probably already know that I did pretty good for that pair of units. I do not recommend the 400T. It will be a secondary unit for my son while we are out. The other great feature about these units is the ability to transfer data wirelessly from one unit to the other while in the field. I haven't tried that function yet, but its one of the reasons I decided to get a pair of garmins that were similar.
good luck.