I've had both and creature by a mile.
Congo has some problems.
The theme sucks. Sound sucks. Too many video modes suck. The under playfield ape game REALLY sucks (it's incredibly easy and stupid), and the software ain't great. Artwork is all around ugly. Beyond that it's a total peach.
Creature looks and plays much, much better and the theme and execution is way awesome. It's a blast to play and much more fun than Congo on my opinion.
I'm baffled why Congo has grown so much in stature (and price) over the past 5 years but in my opinion everyone got this one right the first time around when it was an afterthought with little resale value.