As I recall from reviews back in the day the Impala was considered the higher-performance choice, but I really don't care for the "Shamu the whale" look of those things. Nearly 30 years later the condition of the individual vehicle is going to be the primary concern.

My own preference would be for a mid-1960s Impala SS with some modern upgrades.

If you plan to mod it, id have to think the chevy has way more options. Also, the impala has the LT1 engine, pretty stout & reliable. Theres one issue with them, I think it has to do with the water pump being right over the optispark system or something, but it isnt a huge deal. But at that age all bets are off. 30 years of neglect will ruin anything. 

P.S. If youre looking to make a toy out of it and plan on doing major work, if you can find one cheap without a motor, id consider putting an LS in it over the LT, LT1's arent bad but the LS series is phenomenal. 

IIRC, the Impala SS sold pretty well for a while (for a niche car, anyway), quite a bit better than the Marauder.  There are still quite a few of them around in good shape, and I suspect aftermarket support is probably better than the Merc.  I'd go with the Chevy.  And that dark cherry color, if you can find one, would be killer.

[Not a sedan, but the 2004-6 GTO hits the same market segment.  Didn't bear any resemblance to the original GTO, of course, but pretty good sports car in its own right.  Not as roomy as the Impala but not as wallowy either.]