All-Terrain Bike, hands down - a mid range model (about in the $1,000 range). I've commuted on both types of bikes and rarely had the opportunity to take advantage of the higher gearing of the road bike and they are NOT as robust as a decent all terrain bike. My current ATB is a real tank but it is far, far lighter than a racing road bike I used to commute on - these things keep getting better and better.

Put a decent computer on it (they are not expensive) and follow the instructions to calibrate it - the oddometers are quite accurate IF you calibrate them.

If you do not have really robust rims, consider upgrading them. And the Kevlar reinforced tires are lighter and tougher than ordinary tires. There are many different tread designs now and some of the more aggressive treads are brutal/unsafe on pavement and a real chore to ride thru long grass. Some of the designs are too smooth to be useful anywhere but rock or sand. The good news is that there some superb multi-purpose designs that don't give up much in any situation, doing a decent job in in most any environment - seek advice from local enthusiasts/bike shops.

You can tote an impressive amount of gear with an ATB but off-road maneuvering is severely compromised by a high center of gravity - panniers help, but unless you make your own they are not usually versitile. Someone else here may know of some ready-mades that are "adaptable" to carrying on foot, but I don't.

Find a local nut, er, enthusiast, who rides trails and pick his/her brain for essentials. There are some things you simply cannot expect to repair in the field. For example, last year we had a bike blow out the big end (crank) about 1/2 way thru a grueling 18 mile leg in the mountains. Repairable, but not without the parts. The bike had NOT been maintained, which is an essential part of any serious ride. How did we finish that ride? One of my boys loaned his bike to the rider of the busted bike and then "skateboarded" the broken bike the rest of the way by standing on one pedal and pushing the ground away with his other foot - it was amazing to watch; he maintained an average speed of about 12mph. If it had been a Bug-out scenario, he could have moved the loaded bike better than trying to go on foot.

There are a few tools that you might want to add to a bike BoB that are not easily handled by ordinary tools - like a pair of 15mm slim wrenches (your bike may use a different size - see your local bike mechanic) - but that kind of stuff is too heavy to lug around for simple commuting.

I've seen the kevlar spoke (several local bike shops stock them) and our favorite mechanic/enthusiast carries one and claims they work as advertised in his experience. In any event a spoke wrench appropriate for your bike is an essential.

You can get away with sewing up a ruptured tire on an ATB and re-inflating it - an interior swatch of high quality duct tape over the repair may help preserve the tube a little longer. Usually if you suffer that sort of damage, the tube is not practically repairable, so I would for sure carry a spare tube (and a patch kit for less catastrophic punctures) - btw, plastic tire levers are worth carrying. You will have to baby the sewn-up tire and don't get too eager with the tire pump, but it is doable. No such luck with a 90 psi road tire in my experience.

Whatever multi-tool you have, make sure it will actually make all the adjustments you need. I find a 10mm combination wrench a la Craftsman or SK to be far superior for certain routine adjustments... YMMV. I'm not a fan of bike multi-tools.

Some folks run pre-cut and terminated spare cables alongside every cable on the bike. I think that is a splendid idea and if you're not sure how to go about that, have your local mechanic set you up. And then have him teach you how to adjust the derailiers properly if you are not already certain how to do that. Ditto brakes.

HTH,

Tom