B44:
Pro: Less expensive kit, cheaper parts, wider parts distribution, uses industry standard AE/Losi style wheels right out of the box.
Con: Much less durable, certain easily breakable parts are really hard to find.
D4:
Pro: Much more durable than B44, Parts last longer, can be modified to use AE/Losi style rear wheels with no optional parts.
Con: More expensive kit, Parts more expensive (but you don't break nearly as often), requires optional hexes to use Losi style front wheels.
I chose the D4 because it's a much more durable car, and requires less maintenance. Handling-wise both cars are nearly equal.