Depending how much luggage is " a lot of luggage", you may want to looking into a car service. my inlaws arranged a van through Barcelona day tours. They were great and manage to fit the 6 of us as well as all our large suitcases, carry on and personal bags. We don't travel light so there was no way were were fitting in one cab.
I thought about a car service, and it seems to be 2-3 times the cost of a taxi. We will have 3 adults and 1 6yo (with booster) and probably atleast 4 suitcases plus carryons. Hmm, I suppose we could get 2 taxis, but it may be easier if they actually have a service with a larger vehicle to get everyone in one.