No, I have had quite a bit of experience with heavy equipment, including Excavators and trenchers just not in this area, and not trenching that deep. ( or any with GEO) I Can easily shave $4000 off the cost with the excavator, and about 4300 with the trencher, I just don't relish the thought of a week or more on the Mini-X. The other advantage is being able to trench water and electrical at the same time.

But might just have to bite the bullet and go 7 to 8 feet with the mini X which is where I would like to be....

One would think that the company renting the trencher would have a very good idea of how well it performs in various conditions.

You know, I've rented heavy equipment on literally hundreds of occasions and what I would say is that more often than not the clerk at the rental company has only a passing knowledge of all the machines capabilities and next to no knowledge of your particular job. He is there to rent a machine. What do you think will happen when you ask if it can do the job?

But what's the worse case - you return it after one day?

I think a trencher CAN get pipe in the ground, but, judging from the other questions presented, it is clear the OP doesn't have enough experience with this to be aware of any other things that might be issues and from that standpoint, the excavator seems like a much better idea.