Soundfonts are just sound samples whereas sampletank does have built in effects such as reverb and rotaty speaker and is more of a complete tool but costs.....though soundfonts can be readily downloaded from various sites off the net and reverb could be added by another plugin after it - a poor mans sampletank.
On the sagan link site there are a few websites that house soundfonts.
A really good one is hammersound -
http://www.hammersound.net/cgi-bin/soundlink.pl
Download a soundfont from the site. Avoid soundfonts that mention a site something like
www.melodymachine.com. They are compressed and can only be expanded by a PC utility on a PC.
Ideally the soundfont downloaded will have a name and .sf2 at the end.
To use in Metro bring the plugin DLSMusicDevice onto a track and then click on the none button in the effects window to locate/load the downloaded soundfont.
Some of the soundfonts located in the collection folder/area are good enough to be replacements for an external synth and can have a whole range of instruments and drumkits in one file.
Scc1t2.sf2 in the collections area is only 3MB and is pretty impressive for its size. 8Rock11e.sf2 is 8MB and has slightly higher quality sounds.
If you were to just download a piano and it's very large, you could get multiple velocity settings and the sound could be awesome.
On the other hand you could download a 20MB beast and it sounds absolutely terrible.
Soundfonts are well worth looking into though.