My strong advice is one tree.
When you run an ancestor report or chart for say your children, all their parents, grandparents, great grandparents are available.
All your marriage, parentage and kinship reports will be complete and not half truncated.
If you do a split tree (his and hers) then you also have the additional hassle of keeping two trees synchronised.
With a single tree you can have a single online Ancestor member tree with all your ancestors.
It is dead easy to extract any line out of FTM as a report, chart or file should you need to do so.
Also read this article on the FTM knowledge basehttp://ftm.custhelp.com/app/answers/detail/a_id/2271/kw/more...
To me the jury is back one tree, but others may have different opinions