It's going to come down to a simple equation: style versus substance.

Will buyers of full-size trucks appreciate the dramatic, wide-ranging improvements found in virtually every nook and cranny of General Motors Corp.'s all-new Chevrolet Silverado and GMC Sierra - or will the appeal come down to how good the trucks look against the competition?

Unfortunately for simple marketing purposes, GM's 1999 full-size pickups' most noticeable attribute - the sheetmetal - also is their primary weakness. Not bold like Chrysler's Ram, say some. Not enough sophisticated assertiveness like Ford Motor Co.'s F-Series, say others. Let's see.

Sierra's styling is contemporary and sweeping; the Silverado, more, well, Chevy-middle-of-the road. Both do jettison the boxy headlights and hard edges, lingering treatments of General Motor Corp.'s conservative design past, and open up cab visibility with retro-sized windows.

GM says the new GMT800s' styling was driven by its customers. That's their story and they're sticking to it. The '99 model likely will retain its current drivers, but winning over F-Series and Ram loyalists will be a tough climb even if an avalanche of buyer incentives begins.

If that's the case, GMC's Sierra advertising plan, which will triple in spending this fall after the truck stars in the summer movie Lethal Weapon 4, might be in trouble. That's because it is targeted at so-called "progressive truck buyers" who prefer "substance beneath style."

Chevy, meanwhile, will always sell Chevy trucks just fine. If anything, both versions may enjoy something of a "new conservatism" as some lifelong truck buyers start to get older and view great leaps of styling like the Ram as too intransigent for their tastes.

GM must forge ahead with such a high-profile introduction without the popular fourth-door option. The fourth door is on the Dodge Ram, and it is coming on the '99 FordF-150, but GM buyers will have to wait until next year.

The engines, all up-rated for power yet downsized in displacement, make a fine run of it. The 4800 OVH V-8, in particular, feels alarmingly more powerful than the 5L V-8 it replaces, making 255 hp, a 25-horse jump despite the 200-cc displacement deficit. Never mind the whopping 300 hp from the 6L version, and even the turbodiesel (which remains at 6.5L) makes 25 hp more than the old oil-burner's 190 hp.

The transmissions, as we've come to expect from GM, are top-notch, too. Shifts are neat and clean, and a great new easy towing function holds gears longer to minimize gear "hunt" in hilly terrain.

You want good brakes? On a truck? You've got 'em on any GMT800. Big pie-plate discs at each wheel, clamped by hulking calipers. All-wheel discs are options for the competition, and the performance of the Sierra/Silverado brakes blows away the others' 4-disc setups. And there's great feel, too. The brakes are these trucks' most rewarding new feature.

The wheelbase has been increased by 2-ins. (5.1 cm) on extended-cab models, creating a largely more useful interior. The 10-in. (25-cm) ground clearance is maintained, but the cab floor is pulled down an inch to improve step-in. The interior's all-new and a good deal more comfortable. We are particularly struck by improved seat positioning that makes one feel a part of the truck, not teetering on top.

Designers spent 5,400 hours developing seat comfort. The result: premium bucket seats up front, rear seats with longer (2.4 ins./6.1 cm) bottom cushions and higher (3.7 ins./9.4 cm) rear seatbacks, and seatback angles that have been changed by four degrees from its predecessor to 18 degrees.

And the chassis is delicious - at least for a truck. GM had some old models for comparison, and after driving the Silverado and Sierra, all we can say is the old trucks felt hinged in the middle. Thank the guys who developed hydroforming for a wonderfully stiff and strong frame.

In all, GM's done a thumping good job of updating its bread-n-butter - a task more difficult than we'd like to assume. Everything else - from driveline to chassis to accommodations - needs no qualification.

If true truck buyers buy for substance, the all-new Silverado and Sierra should fear no competitor.