Edited by Alpenglow - 9/19/13 at 5:33pm
If your child is mature enough to sit properly in the seatbelt all the time, then no, there is no documented safety advantage to a harness over a booster at that age.
The other option is to use the frontier, but also invest in a cheap booster (high back turbo would be my suggestion) to use when you need 3 across.
The monterrey is a great booster, but the belt fit isn't always great on narrow shouldered kids. So buy with a good return policy or try before buying.
5yos are often able to use boosters correctly. If yours can, it's a safe and appropriate choice.
However, in tight three-across situations, harnessed seats are often much easier to use than boosters. They may be a PITA to install, but once they're in, they're in. Boosters need to be buckled each and every time, and when they're tightly crammed against the other seats, it's not quick or easy to buckle. (And the Monterey is a very nice booster, but it's also a very wide one. I don't think it's your best choice here.)
I'd look at the Evenflo SecureKid seats. The harnesses are about 1.5"-2" taller than the Marathon, so they will definitely give a 5yo another year or two at the least in the harness, and then it converts to a decent booster. And they are fairly narrow seats and might work in a three-across.
It's outgrown in booster mode when it's outgrown in harness mode. There's just no point to having it as a booster seat. It's like saying, "My vacuum is also a key hanger!". Um. Why? LOL.
And, it doesn't position the belt that well or comfortably either.