Both versions are fairly stable. Most bugs affecting 5.1(5)E1 also affect 6.0(2)E1 and vice versa, since both are running the E1 engine analysis code.

If this is your first sensor installation go ahead and ho with 6.0(2)E1 is my recommendation. The 6.0 train has the latest features like virtual sensors. And new features will come out in later 6.x versions.

You will also be figuring out how to manage the sensor and what management products to use. By starting with a 6.0 sensor you can have better assurances that your choices for management stations now will be supported for much longer than management stations that were geared for 5.x sensors and released a year or 2 ago. You don't want to have to get 5.x up and running, only to have to upgrade to 6.x in a year and redo all of your management station qualification testing.

If you already have 5.1 sensors and have specifically qualified 5.1 to work on your network, then go with 5.1(5)E1. You would already have tested 5.1(5)E1 to work with your existing management station, and putting 5.1 on the sensor would allow you to easily role it into your existing framework.

So if you are a new user I would start with 6.0 and only revert to using 5.1 if you happen to come across one of the few issues specific to 6.0 sensors.

Marco