Bundle a 40gbps interface allow you to use a 40gb link with the device on the other side of the mpo fiber. You must have a switch or router with this kind of interface available.
Using lacp allow you to join several 10gb fiber if you don't have a qsfp+ device on the other end. Also, theoricallity and lacp trunk is really limited to 10gbps for each point to point connection. I mean, lacp protocol choose the trunk interface to use for a flow based on ips or macs. For all the flows from one source to one specific destination, all packets will traverse only one interface so you are limited each interface bandwidth.
I think it's better to use bundle if you have 40gbps capable devices