Crankshaft design. The first V8s were essentially two four cylinder engines on a common crankshaft. The crankshaft has throws at 180 degrees, hence the flat plane reference. This engine offers the best V8 tuning potential. Balance is an issue, often requiring balance shafts.

The cross plane crankshaft has throws at 90 degrees, with two con rods per throw. You need to add heavy counterweights on the crank, so it doesn't rev as well. However, it offers pretty good balance with those counterweights (not as good as an inline 6, but approaching it). This design of V8 was essentially a compromise, driven by wanting the packaging efficiency of a V8, and trading off several other design goals. An example is that the firing order is not even, which results in the characteristic V8 burble, but also means that exhausts have to be interconnected between banks for tuning. All V8s designed for ultimate performance (Cosworth, Ferrari, etc) will be flatplane designs (quite a generalization there).

The biggest driving differences between the two designs will be exhaust note and how quickly the engine revs.

__________________
2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue