Some comparison photos showing a Rollei 35, Leica IIIf with CV 15mm & finder, and a Leica M2 with CV LTM 35mm Skopar (and an M Grip)...
General view
Front view showing widths...
The M2 is a little wider than the IIIf, while the Rollei is significantly smaller.
Fronts, side by side to show heights...
The Rollei is only a little shorter than the IIIf itself, but the latter becomes the tallest with the add-on finder. The M2 is noticeably taller, more so with the addition of the M Grip.
Tops, showing body thickness and lens protrusion...
The IIIf has the slimmest body, with the Rollei and M2 almost the same. Lens protrusion is least with the Rollei by a small amount, and virtually the same for the Leicas, although this will obviously vary depending on choice of lens. The 15mm has hood-type features built into the lens barrel, while the 35mm Skopar has a shallow screw-on hood.
Weights as shown...
Leica M2: 780g
Leica IIIf: 600g
Rollei 35: 350g
In terms of handling and usability, the M2 eclipses the other two by a long way, even though it's noticeably heavier and bulkier. The main things that make it so usable are the M Grip and the right hand lever film advance. It also has the best finder (big and clear with a bright RF patch), although the Rollei is a close second. Both have bright lines for framing, with the M2 going out to 35mm. The IIIf's built-in finder is a bit of a pain - small with a 50mm-ish field of view, no frame lines, and with a separate RF window (which has a decent patch, but not as good as the M2). The add-on finder is big, clear and bright, but has no frame lines.
The IIIf and Rollei are both quirky in terms of handling. The IIIf has knob film advance and a lifty-uppy shutter speed dial for speeds 1/25th and faster, and the little dial on the front for slower speeds. The Rollei has the shutter and aperture settings on the dials on the front, which are quite usable, but the right hand dial gets in the way of the fingers somewhat when taking photos. It doesn't move off the setting (it locks), but there is very little to get a grip of because the camera is so small. The left-hand film advance lever is okay other than needing to take the camera away from the eye every time you advance the film.
For film loading, the M2 is, perhaps surprisingly, the easiest, at least when standing up. Although it's a bottom loader, the fact that the camera hangs on a neck strap makes it all pretty straightforward. The Rollei is more conventional, but you need to be sitting down, preferably at a table - a bit fiddly when standing up because you have to hold the camera in one hand. The IIIf is also a bottom loader, but without the back door on the M, and the film leader needs to be cut to make the narrow bit longer. I don't actually find this too much of a hassle when standing up - I always carry a Swiss army knife and trim the leader using the scissors by eye (the length isn't particularly critical). Again, loading the IIIf while standing is helped by the camera hanging from the neck strap - both the Leicas can be flipped over so the bottom is uppermost with the lens against the chest, so they just sit there and both hands can be used to do the loading.
I've used all three for fast and loose street photography (ie, in very busy situations with lots of people around and lots happening), which is probably a good test of usability when you don't have time to faff about. In spite of its greater bulk and weight, the M2 is easily the best - the grip and right-hand advance lever mean it can be readily used one-handed if desired when zone focussing. The 35mm Skopar has a focus lever and that helps a lot if using the RF for focussing (very quick in conjunction with the excellent RF patch). The bayonet mount is much quicker for changing lenses. The IIIf is fine if used with a wide lens and add-on finder and zone focussing, but too slow with the more typical 50mm and using the RF to focus. Exposure adjustments can be fiddly when in a hurry, and the shutter speed dial is partially obscured by the finder (some add-on finders might be smaller than the 15mm one, but not by much). The Rollei 35 is probably a bit easier to use than the IIIf, or perhaps fiddly in its own way - exposure adjustments are easier, but the left-hand film advance means you're always moving the camera away from the eye between shots.
Overall, for serious picture taking, the M2 is the first choice, but it isn't a pocket camera (or even a compact in my view). It has the best ergonomics by far, and is fast and intuitive to use. At the other end of the scale, the Rollei 35 wins easily for carry everywhere pocketability, but falls down a bit on ergonomics and doesn't have a wide lens. There are other compacts that are almost as small (eg, Olympus 35RC) and have better handling, but still tend to the more moderate end of wide angle. The IIIf is reasonably compact for a jacket pocket with the collapsible Elmar, but not with a conventional lens and add-on finder. The controls are a bit weird, being a gradual development of the original Barnack Leicas where bits have been added on rather than the overall design being revised. It is good fun, but you either need to set it up for a particular use (like 15mm zone focussing and don't fiddle too much with exposure settings), or not be in a hurry.
I can't think of anything in the size class of the Rollei that has a wide lens. If your M6 has one of the wider finders, it will have 28mm frame lines, and the finder itself might get to 25mm or so. The M2 has 35mm lines and might get to 28mm at the frame extents (albeit without parallax compensation), but it's hard to see the frame edges all at once (eye gets very close).
