Exchange 2016 and VMDK or in-guest iscsi

Hi, I am working on virtualizing exchange 2016 and was wondering if there where any suggestions as to whether to use VMDK or in-guest iSCSI.  Our storage vendor recommends in-guest iscsi and our backup vendor says vmdk is fine.  I realize that using in-guest would eliminate Veeam for backups at this time.  All the other VMs in our environment are using vmdk, so this would be a one off if we were to do in-guest iscsi.  Any advice would be appreciated.

Thanks,

Bill