Should I purchase the German-Poland Railpass or just purchase the Single country German Pass and pay for another ticket into Poland/
The only way to reasonably answer the question is to know when you are traveling and what points you are traveling to and from. The conventional wisdom on this site seems to be that point to point tickets may be cheaper. You may want to read what Rick says about rail passes and also look at the man in seat 61 website.
There isn't really a clear cut answer about rail passes. Unlike years past when rail passes were a no-brainer, these days it just comes down to doing the math to see which is the better value. We typically recommend adding the cost of the point-to-point tickets and compare that to the cost of the rail pass. One is usually cheaper than another. I hope this helps some.