IMO, you really need to book the Church first. What is more important?
Also, what if you book the hall and then you can't get that date or time at the church? people do this all the time and wind up having a lot of problems beause they either have to look for a new church, or have long gaps in between the service and the reception, etc. etc.
There are a lot more halls out there while you only have one church. and also, Catholic churches usually only marry parishoners so it may be harder than you would think to find another one.