I've used both Fink and macports
I used Fink for a few years from 2002-2005, back then it then there was no macPorts (it was called Darwin ports). The main reason I used Fink was because it provided binary distribution. This was a must on G3 with 512Mb
Fastforward 2008. I got an intel based mac. Fink had only outdated binary distributions for my platform. So I moved to macports, since I'm going to be compiling everything from source anyway.
I've found some difficulties installing some ports from Macports with a fresh install (mainly due to playing around with variants or conflicts with libraries installed by apple). Otherwise it has been fine and the community is responsive.
I haven't tried fink since but it seems that many of the ports in the stable tree are outdated.
However, there is no reason to install both on your system. The only problems that may arrise are
- Duplicate installations of the same libraries
- Related to 1, but you might get conflicts between libraries installed by either method depending on which one the conf script finds first. However it is the same problem with apple provided libraries.