As for functions, by now everyone in the cyclecomputer biz seems to have settled on the basics: Distance, time, average speed, maximum speed. Myself, I want to know how fast I'm going at any moment, how far I've gone, and my average. I do like the little up/down arrow some computers have that show whether you're above or below your current average speed.
Anyway, one strong candidate would be the VDO C3DS ($90; available at www.nashbar.com). It's a fairly high-end wireless model, and can be swapped between two bikes. You can maintain separate records of trips and distances. It even has a feature that lets you count distance up or down, an aid when using maps that, say, tell you to turn right in 6.7 miles. And it's waterproof.
CatEye's MC100W offers many of the same featureswireless, two-bike compatibilityin a slightly stripped down and more affordable package (it's $50 at www.cateye.com, though is currently $40 at Bike Nashbar). It doesn't have the spiffier VDO's navigation function, but it has all the other must-have bike-computer features. And it has the pace arrow I mentioned. I've long used CatEye computers and found them to be exceedingly reliable.
In a wired computer, the CicloSport CM211 (www.ciclosportusa.com) gives you two-bike capability for $40 (or $20 at Nashbar!). The only question is how the pickup will fit on a suspension forkyou may have to rig something with zip ties, but that's common enough.
Of course, for less than $50 you also can simply buy two computersa CatEye Mity 8 for the road bike and an Enduro 8 for the mountain bike (each $20 when on sale, which is often) and never have to worry about it! I have computers on all my bikes for just that reasonno fiddling around.
Pick up a copy of the 2006 Outside Buyer's Guide, on newsstands April 25, for a look at 396 torture-tested products, including the 2006 Gear of the Year award winners and an all-new women-specific review section.