I bought EQ6, because it was already out when I started quilting. Later I bought the Dear Jane software, which is based on EQ5. I can see a big difference in user-friendliness between the two versions, and EQ6 also has more features.
You have to be careful when purchasing used software, because EQ can be installed a limited number of times. This policy protects against software piracy, or sharing, which is the same thing. I don't view the limited number of installations as an issue if I am buying new software, because the manufacturer will make allowances for legitimate need for additional installations. However I would view it as a problem with previously owned software.
The other things to consider are support and upgrades. I am looking forward to the day when EQ7 comes out and I can get even more features by upgrading from EQ6. EQ typically charges half the price to upgrade if you have the most recent version.
There are online retailers who discount EQ6 so it comes to around $130 with free shipping.