Which relay will last the longest?
Interesting question.

In terms of switching there is no real difference. When switching a contactor/relay one contact will always make or break first/last. Or to put it another way, only one contact actually does the switching. That is why you should never double up contacts to try to turn 30A relay into a 60A unit.

So, what else is there?

Contact resistance. When closed the circuit on the right shares the load current equally through both contacts and will therefor drop half the voltage and heat less. In the left circuit, both contacts must carry the full load so you end up with twice the voltage drop compared to a single contact.

There is one more factor. Isolation. In the left circuit when the contactor is off you have full isolation between the load and the supply. The right circuit you have only disconnected one line. The left circuit is therefore safer.

So in other words it comes down to do you prefer isolation versus quarter the voltage drop and a cooler contactor.

ADDITION:

Since folks brought it up in the comments, here are the fault scenarios, not including coil failure. 



From this it is evident the series connection is possibly better if you need it to fail off.