It is NOT a question of easier to install, it is which one can your hull
design support. Both of these designs are taking the vertical pull of the rigging
, and converting it to horizontal loads.
The bow tube is under significant compression
, and because of this it takes the rigging
loads that are trying to pull the bows together. Rigging loads on the hulls at the bow are relatively small.
The "longeron" accepts none of the load that is trying to pull the hulls together, the hulls have to resist that themselves, and these loads are VERY large. It does present the hulls with some backwards loading at its attachment point midships, but this isn't very big. With no rigging supporting the forward end of this spar, it needs to be extremely strong and stiff to support a sail out at the end.
One is not "better" than the other. It depends on the hull design and what loads the hull can accept. Certainly which one is easier to install should not be the primary criteria.
This is not a decision that is best made at the end of the design process. Either the hulls can accept the inward loading, or they can't. If you don't know, the bow tube is probably the safer choice.