The trencher you rent should be sized for the size of the job. If there is a lot of wide open, long runs, get the ride on type, if there is a lot of tight areas, trees, and short runs, a walk behind is the ticket.
If you have never used a trencher, or put in irrigation, get something smaller so you can learn as you go. The vibratory plows are great in sandy loam with no tree roots and rocks. Again, you should have an idea how the systems go together before using the vibraplow.
I would get a small trencher, take my time, dig plenty deep (24") and learn how the systems go together. After putting in a few systems you can worry about other trenchers, vibraplows and the like. If you put many systems in it pays to own the trencher. We were renting 2 to 3 days twice a month @ $150.00 a day. I bought a Vermeer V1850 walk behind for $11,500.00 with payments of $310.00 per month.
Good Luck,
Greg Pierce, CTP