Hi Clement. According to me the main points which need to be considered are as follows:

(1) The main advantage of wizard.interface is we can generate no. of reports & call a different wizard at the same time which is not possible in osv_memory wizard.

(2) Wizard.interface is not compatible in 6.0 later versions.

(3) In wizard.interface there is a flexibility of validations. That makes validation part separate but still performance wise it is not as good as osv_memory wizard.

(4) The main advantage of osv_memory wizard is you can define workflow on object. We can easily inherit osv_memory wizard. It is very easy to make changes in view of wizard by just inheriting it.

(5) We can add constraints in osv_memory wizard.

(6) Another difference is we can not set default value in interface wizard in client(right click on field of wizard and setting default value) whereas in memory wizard we can set.

So overall it is better to use osv_memory wizard.