Hey Woodsie,
There is no shortage of trails in the Zurich area. Uetliberg is perfect for after work, even if you are on the wrong side of the city. I work near Zolikon, and regularly ride over to Uetliberg and do the trails there before riding back to pick up the car and go home.
There are also some good trails around the Zuriberg side of the city, with a cross country style loop and a bike park on top of Zuriberg itself. There are some trails in this region too that you can link together to ride all the wayup to Pfanensteil which will put a little distance and climbing into your legs if your feeling energetic after work.
My personal feeling is that a light full suspension bike with a decent amount of travel is the best option for around Zurich. There is a fair bit of climbing to do so a full on downhill bike could be a touch heavy, but some of the trails have some pretty interesting sections and jumps (if you want them) so a nice bit of travel front and back doesn't go astray.
If you ride the Pfadiwegs that Frasier was on about you want to be carefull of hikers. They are not a huge fan of bikers flying past while they are out walking the dogs... but fortunately there are enough singletrails around that you will probably only need to climb on the pfadiwegs anyway.
Check out the site
www.singletrailmap.ch for maps of each area that show some of the trails. Then go searching and find some more for yourself!!!
You will also find that there are groups that meet most evenings of the week to go for rides. These can be a good way to get to know the trails and meet new people.