I have no olli to compare it to, but here's my thoughts.
the olli is easier, most likely, to install/uninstall if you're going to be moving it a lot, because of the rigid latch.
The Monterey is much deeper, but the olli is known to be comfy, despite not being very deep. Still, for my kids who do care about depth, they love how deep the Monterey is.
I don't know which is wider, but the design of the armrests would make the olli easier to buckle (the armrest design took a little getting used to for tightening the seatbelt for Jeffrey), but both seats are nice and wide.
And, though they only matter to the kids, the Monterey has 2 cupholders (really great ones), but the Olli doesn't.
Really, I think a lot depends on what you need as a booster.
I was between the Olli and the Monterey and how I decided was to let Jeffrey choose. The reason? He could fit in the Monterey as a highback, but, at the size of outgrowing most highbacks, and being a good booster rider, I was comfortable with him choosing whether or not he would stay in a backless or go back to being in a highback.
HE chose the Monterey... he said he preferred the color, but that the big reason was he wanted the side supports (once he was assured they would widen to accomodate his growing bod) to hug him and the headrest and wings (he called them pillows) for sleeping comfortably.
If a child was too tall for the Monterey with a back, I doubt they would need a backless... even my 8yr old giant who is over 4'9 and fits the seatbelts well and passes the 5-step test AND has feet on the floor flat had 1" to go in the Monterey before his shoulder would be at the place where, when I put the headrest that way on his brother, the belt twisted.
Not all cars can work with highbacks, or have the room for a highback, so that's why I'm glad i can convert.
but if getting either one for just a backless, I'd probably not spend the extra money on the Monterey just because in the end, the two seem equal.
Hopefully, we'll see them soon selling just the bottom part of the Monterey for kids who need them, and they will be a cheaper alternative to the Olli, but who knows.