Bootp is the original forwarding method and the dhcp helper options are a later addition to Junos with more configuration options for control.
If your needs are simple and you don't use routing intances with dhcp, I would stick with the bootp version.
You cannot mix the two on the same box. And bootp option does not work inside of a routing instance so you would need to use the dhcp helper version then.
Dhcp helper is also better if you are using a subscriber management system for dhcp as it can be modestly stateful on the MX and work hand in hand with such a management system.