I am looking to fit an oil breather/ catch tank to my FJ20 turbo, and notice some come with a "breather" or filter attached while some seem to be a sealed unit. Does anyone know if the FJ engine is better suited to sealed or unsealed?? Could either cause +/- vacuum etc.??

There is normally a connection from a "sealed" unit to the intake manifold. The crankcase emissions are consumed by the engine in that way.

A truly sealed unit would not provide a realistic flow path for vapors to even reach the catch can.

If you run a filter equipped unit, particularly in a forced induction engine, be sure that the filter does not become wetted out by oil. This increases the pressure in the crankcase and oil will then be encouraged to find any possible route to escape, generating leaks. Also, the more atmosphere present in the crankcase the more windage effects that will be present. In hard use this can lead to oil foaming and potential engine damage from a variety of means.

So you think a unit without a vented filter, but with a return line to the manifold would be the safest solution?

Yes, and I am presuming there is already a location for this on the manifold. PCV.