When shopping for computers, buyers are often presented with a baffling range of seemingly similar products at widely varying prices. Just what do you get for the money if you choose to pay extra? The answer is not always clear, since the choices may involve complicated and sometimes subtle trade-offs.

On the surface, the Lenovo ThinkPad T400s and the Acer Timeline 3810T seem to be fairly similar laptops in a category often called "thin and light." The ThinkPad has a 14.1-in. display, is a hair under an inch thick, and weighs 3.9 lb. The Acer has a 13.3-in. screen, is 1.1-in. thick, and weighs 3.5 lb. Both run on Intel (INTC) Core 2 Duo processors with the same Intel graphics adapter, and both displays use energy- and space-saving LED backlighting. But the Acer Timeline retails for $900 while the ThinkPad starts at $1,600.

Part of that difference represents extraordinarily aggressive pricing by Acer, whose top priority in the U.S. has been to build market share rather than earn a profit. (It's working: Acer is poised to become No. 2 in PCs.) Given the components used, I think it's likely that the Timeline is being sold at a loss. Although the Acer outdoes the ThinkPad in one important category—battery life—the T400s seems superior in most others. Several considerations may help you decide whether that superiority adds up to $700.

T400s Doesn't Disappoint

Some of the most important differences don't show up in any list of specifications. ThinkPads have a well-earned reputation for rugged construction and great keyboards, and the T400s does not disappoint on either score. I have the sense that an elephant could dance on the lid without causing damage, while the new Acer has a somewhat flimsy feel.

The ThinkPad's keyboard is outstanding, with sculpted and well-placed keys and the choice of a pointing stick or a textured touchpad for controlling the cursor. The Acer's keyboard is by far its weakest feature. The keys are flat with an oddly slippery surface, and the single button below the touchpad rocks for left or right clicks but doesn't provide good tactile feedback.

There's always a trade-off between battery life and performance, and the two companies made different choices. Acer went with a 1.4 gigahertz ultralow voltage processor; Lenovo chose a 2.4 GHz standard-voltage CPU. I tested battery life by leaving the laptop with its default power management settings and playing a video continuously (grueling, but somebody has to do it). Acer's Timeline ran for just over five hours—well short of the nine hours claimed, but still impressive. The ThinkPad pooped out after 2½ hours, though you could extend that to about four by replacing the DVD drive with a second battery. But the ThinkPad ran rings around the Timeline on a taxing performance test: It was 50% faster at converting a video to iPod format. Whether this speed difference is important to you depends on how you use your computer. If you just write e-mails and view Web pages, you probably won't notice the difference.

Acer's Cheesy Keyboard Hurts

The ThinkPad is designed as a corporate workhorse and has features, such as a connector for use with a dock, that an individual buyer is unlikely to use. It also includes a considerable amount of utility and management software that is of more use to IT professionals than consumers. Business buyers should find this an attractive notebook.

The Acer Timeline offers a spectacular amount of computer for the money. Were it not for that cheesy keyboard I would put it close to the top of the list for value among notebooks I have seen this year. Keyboards and touchpads are a matter of personal choice, so this is one computer you'll want to try out in a shop. If you can live with the keyboard, it's a great buy.

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