8:03 p.m. on August 26, 2012 (EDT)
Which to choose depends very much on how you intend to use the widget. Too bad you have settled on (1) Garmin and (2) the Montana series. There are better units out there. But, since you have locked in, I suggest you read the reviews by GPS Tracklog:
Montana 600
Montana 650
Montana 650t
The big differences are that the 650 adds a camera (which geotags the photos) and the 650t includes 100k series topographic maps. If a camera that geotags is important to you, then get either the 650 or 650t. As for the maps in the 650t, for one thing they are 100k maps (small scale as you would expect from being 1:100,000 scale, which means the details appear very small on the 4-inch screen). There are several sources of downloadable 1:24,000 maps available for less than the price difference (1:24k is the standard scale for USGS quads). Depending on what you are doing, the standard basemap may be adequate on the 600 and 650, and if you need more detail and larger scale, you have the option of loading 1:24k maps (I suggest one of the 3rd party vendors, since I find Garmin's 1:24k to be expensive and not as good - besides you really need a paper map anyway - no running out of battery and a lot easier to use).
As for the camera on the 650 and 650t, The spec says 5 Megapixel, pretty poor when you compare to almost any P&S digicam on the market today. It makes for a very expensive substitute for a real camera.
The Montana, like the Oregon, has a touchscreen. At this point, I have found the touchscreens on GPSRs to be pretty poor, especially both the Oregon and Montana. "Touchy-screen" would be more accurate. It is often hard to get the screen to do what you want (I would suggest you give any of the 3 a thorough test in navigating the commands and screens before you lay out the money to get one - some people love touchscreens, even though the screens get mungy pretty quickly, especially when sweating on a hot day hike). Garmin claims the touchscreen can be used with gloves on. I found that even with special touchscreen gloves (I have 3 pairs from different companies) it is hard to get to locations near the edges and especially corners of the screen. Getting the touchscreen to work with winter gloves on is pretty near hopeless (same with other companies' touch screens, including Smartphone navigation screens). A touchscreen stylus works much better.
Also be aware that the minimum operating temperature is -4F/-15C. That may be fine for you, but I spend a good part of the year in temperatures much below that. Garmin does not specify a maximum operating altitude for the Montana series. Since I got to altitudes above 10,000 ft and up to 20,000, that is a serious concern for me.
Another serious concern (for me, at least) is Garmin's insistence on locking you into using the barometric altitude only, with no way to display the GPS-derived altitude permanently on any of the windows. Again, I know there aren't that many people who go to polar regions like I do, but since the pressure-altitude relationship is very different in polar regions, the GPS-derived altitude is vital (on Mt Vinson, for example, the geometric altitude, which is available derived from GPS signals, is 16,000 ft, while the barometric altitude is typically more like 19,000 ft, depending on the current weather situation).
If you were willing to consider alternatives, and were getting close to upgrading your cell phone, I might suggest looking at the smartphones like the iPhone and Samsung's Android phones. Smartphones these days all come with GPS chips in them (same chipset used in the iPhone and Samsung as Garmin uses). There are a number of excellent mapping apps that provide highway routing to get you to the trailhead and topo maps of the trails once you get there. This includes scanned USGS quads, local area trail maps, and many more.