As I tell myself everyday after I drive my S 500 L: there is nothing like Mercedes-Benz V-8 power, period.
Just on this issue alone, you should get the E420!
BUT, you may consider a couple of interesting facts regarding this model:
The W124 M119-engined E420 was produced almost exclusively for the U.S. and Canadian markets to satisfy the urgency of more power and capacity (compared to the E320). Hence, it became a real limited number model.
M.B. factories in Europe could not fit such small numbers for a special niche model in their lines, so basically what they ended up doing was shipping unassembled W124s bodies and fully assembled M119 engines to Mexico.
In Mexico, at a Mercedes-Benz truck plant or the VW plant in Puebla (not sure which), workers will put together the W124 bodies and fit the M119 in it.
So purists consider this a total sin, preferring the E320 over the E420 just for this fact. Also, some E 420 owners complained about fit and finish problems (that the euro E320s did not present).
The other factor to consider is the the W124 chassis was never intended nor design in principle to carry a V-8 engine. Euro models had a wide range of 4, 5 and 6 cylinder engine options.
The fitting of the V-8 generated some cooling, suspension, and engine mount issues on the E 420 and E 500 W124s.
I am NOT trying to rain on your parade or anyone else, just wanted to share a couple of facts that you might consider important to make your purchase.
CL 500, 1998
S 500 L, 1998
E 320 T, 1995