I decided to take matters into my own hands today and went all around town to find these different Mountainsmith bags instead of packing my apartment like I should have done. My comments on these bags all relate to taping purposes, not hiking or whatever they are intended to do. I ended up purchasing the Day.
The main difference between the Cairn and the Day is that the Cairn is shorter and deeper while the Day is taller. The Cairn has a padded back and a medium-sized padded front pocket which would be perfect for a D8. The Cairn also has a bit of a smaller main compartment and a larger front pocket. It has a limited range of how wide you can get the main compartment to open with the zipper; that is a minus in my book. It is rated at 1100 cubic inches. I really liked the extra padding on the Cairn, but it was too small for my very large, old-school Mac G3 WallStreet laptop. If I was only using it for taping purposes, I would probably have gone with it because of the extra padding.
The Day has a padded back part, but no padding on the front. It has internal dimensions of 12"x12", but my computer which is 13" across fits in there very nicely without the bag bulging or anything. You can get the main compartment to open all the way with the zipper; another factor that swayed me in its favor. The Day is rated at 1150 cubic inches, about the same size as the Cairn. I have not used it in the field yet, but I can fit my MiniMe, tupperware container for the Gefells, 15' mic cables, 6 volt EcoCharge, AT shockmounts, SignalFlex clamp, and Shure vertical bar in the main compartment, my EcoCharge and 7-pin cables in the inside smaller compartment, and my windscreens (without boxes, in a Ziploc bag) in the outside compartment.
Both the Day and the Cairn have a bunch of accessories one can purchase, including a padded pouch that I will probably buy for my D8. One thing that pisses me off is that shoulder straps to wear any of the Mountainsmith lumbar bags as a backpack costs an extra $25, which should be included for no extra charge IMO. I will probably pick up a pair for Georgia anyway.
The Tour is smaller than both the Day and Cairn. It is rated at 650 cubic inches and is basically a big fanny pack. The gear I listed above would not have fit in the Tour, but I have a feeling it would be perfect for a stealth or FOB Neumann / DPA / Schoeps rig. It may be able to fit a MiniMe and some stealth mics in there, though. Although the Tour and the Day are part of the same family of Mountainsmith packs, I felt that the Tour had more in common with the Cairn, with the exception of the added padding on the Cairn's front.
I don' think that any of these bags is padded on the bottom, but they do have thick nylon all around. If you are concerned about padding on the bottom, you may want to see if you can put some foam down there (available at Michael's craft stores). None of these bags are waterproof, but they are water resistant. I would most definitely line it with a trash bag or thick plastic before setting it down on the floor of a taper section. Great Outdoor also had these cool large waterproof map protectors ($20) that I will probably pick up to provide some extra waterproofing.
I first went to REI and they had the Tour for $75 and the Cairn for $90. REI also makes a lumbar pack of their own that is 850 cubic inches for $50; it is a more affordable option than the Tour, but does not have the Cairn's padding. I went across town to Great Outdoor Provision Company and bought the Day for $80. These are probably not the best prices available, but they were in stock. All the bags come in red or black / charcoal; the Tour also comes in blue.
Much thanks to Mark and Tim for planting this idea in my head in the first place. I am never going back to that damn ball and chain Rakgear; the Day is 1/3 the size and I have a feeling it will be much more manageable.