I have the choice of using a Debian package and install it via to install it.
To make the latter work I installed the necessary development packages of My SQL. My question is: What is easier in the long run: using the language's specific package manager like gem for Ruby, CPAN for Perl, etc.
The package from Lenny at least used to behave a bit differently from upstream to accommodate the Debian changes.
For doing so I have to install some gems like the ruby database driver for My SQL.
The rubygems approach brings you more flexibility, as you can install any package you like and can have multiple versions of them. If most (or all) of your tools are available as debian packages and the versions suit your requirements, you should go that way.
Mixing both sources is going to cause some serious pain.
If you intend to use rubygems, you should consider installing rubygems itself from source.
Using deb packages as the advantage of having to rely on just one package manager and getting (security) updates for free.The downside is less flexibility, as you can have only one mandated version of a gem installed.