SIP trunking will basically just give you inter-zone calling capabilities and nothing else... No BLF, resiliency, or any clustering features.
Using an enterprise license and IP trunking can basically make the systems one big system (with some caveats) which also opens the door to license sharing, resiliency and failover capabilities and so much more.
Honestly, for two Mitel PBX's I would never use just SIP trunking, far to limited and these things work together so well. Even if you don't think the customer will use these features now, they likely will in the future.
For us, they are either purely stand alone, or their are enterprise and networked... we don't consider any other option.