How is a 30 days novena to St. Joseph different from a consecration to St. Joseph? How is a 30 days novena to St. Joseph different from a consecration to St. Joseph?
Do they have different purposes?
A great question for the Year of St. Joseph.
Conescraton to St. Joseph
Fr. Donald Calloway's book Consecration to St. Joseph essentially is a 33 day Novena ending in a Consecration.  He's the priest who is also wrote 33 Days to Morning Glory, which is an updated version of a the works of St. Louis DeMontfort leading to Total Marian Consecration.
According to the Militia Immaculata Marian Consecration is:


an invocation;
a request that she may deign to accept us as her property;
a supplication that she may deign to make use of us to conquer other souls to her.


Consecration to St. Joseph is in the same vein, except with a special emphasis on St. Joseph qualities which complements his wife The Blessed Virgin Mary.
There are a number of formulations of consecration to St. Joseph in the book, reading the book and doing the 33-day's of spiritual exercises leading up to the consecration makes it all them more understandable to you when you get there.

St. Josephs' Thirty Day Novena
The 30-Day Novena to St. Joseph, which my wife's done an number of times for lots of reasons is a great novena, I think most pious folks consider it an inflatable novena.  But basically it is just 30-days of prayers that reflect on the life and virtues of St. Joseph.
There are some wonderful insights in that book (and Fr. Calloway's book) so I'd recommend doing both this year!

They do have different purposes though, the novena is for a particular purpose (a prayer of petition) and the consecration is for interior conversion (a prayer of supplication).