6yr old travel..BB or RSTV

We are taking my soon to be 6 year old to Disney at the end of the month. He is 49#, 48.5" tall. At home he is in a Graco Nautilus (still 5 pt harness) or a BritaxFrontier 85 Harnessed. He likes to be harnessed normally (he is a very safety conscious guy, lol!) and has only ridden a few times with a safety belt/booster.
We will be once or twice in a taxi, on a few shuttles, driving 2 hours north in my Aunt's car and 2 hours back to Orlando in a rental car.
I would not like to check his nice car seats on the plane. I am thinking he will probably be asleep on one of the two hour trips (hard to keep upright?). My Aunt may be able to bring a safety conscious mom's Evenflo high back booster for her car which might solve my problem with him falling asleep in a booster. Other trips will be daytime/shorter.
Would we be better getting a BubbleBum for taxi/shuttle use or better to spring for the Ride Safer Travel Vest?
In the next year or two he might travel also to Denver and Israel (where drivers are NUTS!), but if we have to buy a size small and he is so big already, use might be less before he outgrew. Not sure if since he is closer to the upper limits if I cam buy a large now?

I am very safety conscious for car travel and admittedly a bit freaky about it!

Thanks,
Andrea

Either one, provided he'd sit properly. I'd probably do the BubbleBum so you don't need to worry about sizes over the years.

Wendy

I think we might order the Bubble Bum and try it out on short trips and see if he will stay upright and sit properly. Since he has never been unharnessed really before, I have to try it out first. He is a good kid though, and if I tell him how to sit to stay safe, he will!
Andrea