Chemically the drugs are very simmilar. If you had to choose between both based on price and effectiveness, nolva should be your choice.
As for when to take it, they can be used during cycle as an ancillary drug to fight off gyno. The usual dose is 50mg of clomid per day, or 20 mg of nolva per day.
For pct the drugs must be started for this treatment only once levels of supraphysiologic test has fallen below natural levels. Most users fail to correctly calculate this, and under shoot by about 3 or 4 weeks.
pct therapy should be continued usually for about 3-4 weeks. Then you should have your blood levels evaluated to see where they were aposed to the beginning of your cycle.
It is a good idea to get your test levels checked before you begin the cycle so you can compare them following pct to see if you have fully recovered or not.