Good call on the Go Hybrid. I thought there was something like that. It would be safer than a Bubblebum since your son, at best, would be at the minimum. The only problem is the cost. You'd also want to double check the shoulder height of the harness, since that's where most children outgrow seats. You wouldn't want to use it in harness mode since that kind of defeats the purpose of buying such an expensive seat, only to use it as a booster.

The RSTV would be easier to transport and cheaper.

Please never use car seats from a car rental company. They're notorious for being old, dirty, out-of-date, etc. Or, what happened to me as a stupid first-time mom, they ran out. I had NO options on car seats for my one-year old. They may also only have forward-facing options, if they didn't run out, which is much less safe than rear-facing (rfing is 11 times safer).

Perhaps Carseat.org would have other suggestions??